Add category list to charms

Add category list to charms

Added category list to charms to support charmworld, which will display charm
categories.  While most charms will have only one, some will fit into multiple
categories, such as Apache, which is both a web server and a proxy.

There will be an official list of categories, but supplying an unofficial
category is a lint error, not a syntax error, so this branch does not address
that.  The order of the values is not significant, so in Python terms
categories are a set, not a list, but since none of JSON, BSON and Go has
direct support for sets, a list is used.

This branch also adds tests to ensure that Meta objects can be roundtripped
via BSON.

https://code.launchpad.net/~abentley/juju-core/categories/+merge/152925

(do not edit description out of merge proposal)

[abentley-home, 2013-03-12 17:58:27]

Please take a look.

[dave_cheney.net, 2013-03-13 00:37:54]

On 2013/03/12 17:58:27, abentley wrote:
> Please take a look.

I thought format 2 was being eliminated ?

[abentley-home, 2013-03-13 12:52:29]

On 2013/03/13 00:37:54, dfc wrote:
> On 2013/03/12 17:58:27, abentley wrote:
> > Please take a look.
> 
> I thought format 2 was being eliminated ?

Sorry, I wasn't aware of that.  What format should I use?

Aaron

[fwereade, 2013-03-15 15:00:37]

On 2013/03/13 12:52:29, abentley wrote:
> Sorry, I wasn't aware of that.  What format should I use?
> 
> Aaron

Just drop explicit references to format in the new charm, please. The explicit
`format: 2` will effectively be ignored, so you may as well leave it out.

[fwereade, 2013-03-15 15:00:52]

On 2013/03/15 15:00:37, fwereade wrote:
> On 2013/03/13 12:52:29, abentley wrote:
> > Sorry, I wasn't aware of that.  What format should I use?
> > 
> > Aaron
> 
> Just drop explicit references to format in the new charm, please. The explicit
> `format: 2` will effectively be ignored, so you may as well leave it out.

Otherwise LGTM, I forgot to say.

[jameinel, 2013-03-18 07:20:46]

LGTM

https://codereview.appspot.com/7762043/diff/1/charm/meta_test.go
File charm/meta_test.go (right):

https://codereview.appspot.com/7762043/diff/1/charm/meta_test.go#newcode317
charm/meta_test.go:317: }
Yay!

[jameinel, 2013-03-19 13:31:25]

*** Submitted:

Add category list to charms

Added category list to charms to support charmworld, which will display charm
categories.  While most charms will have only one, some will fit into multiple
categories, such as Apache, which is both a web server and a proxy.

There will be an official list of categories, but supplying an unofficial
category is a lint error, not a syntax error, so this branch does not address
that.  The order of the values is not significant, so in Python terms
categories are a set, not a list, but since none of JSON, BSON and Go has
direct support for sets, a list is used.

This branch also adds tests to ensure that Meta objects can be roundtripped
via BSON.

R=dfc, fwereade, jameinel
CC=
https://codereview.appspot.com/7762043