Support EC2_ env variables

Support EC2_ env variables

Allow the user to specify their auth credentials using EC2_ style
environment variables. I ran go fmt and it also made some changes.

The tests failed to run due to a test http server not being closed when
each test suite finished. I added a fix for this also, so now all tests
pass when go test ./... is run from the project's root directory.

https://code.launchpad.net/~wallyworld/goamz/ec2-env-credentials/+merge/164848

(do not edit description out of merge proposal)

[wallyworld, 2013-05-21 07:20:30]

Please take a look.

[jameinel, 2013-05-22 11:41:21]

LGTM

[Danilo, 2013-05-22 11:56:52]

LGTM.

It's weird to find a different approach to cleaning up the environment before
the test here and in goose (where we explicitly remove all the env variables we
know the code reads, instead of the os.Clearenv).

This is indeed simpler, but being explicit is usually better.  At the very
least, I'd like us to have a policy of what we do in general (and I know this
was in there even before your changes, just remarking on it).

[wallyworld, 2013-05-23 02:57:47]

Please take a look.

[wallyworld, 2013-05-23 03:14:56]

Please take a look.

[wallyworld, 2013-05-23 03:22:11]

*** Submitted:

Support EC2_ env variables

Allow the user to specify their auth credentials using EC2_ style
environment variables. I ran go fmt and it also made some changes.

The tests failed to run due to a test http server not being closed when
each test suite finished. I added a fix for this also, so now all tests
pass when go test ./... is run from the project's root directory.

R=jameinel, Danilo
CC=
https://codereview.appspot.com/9545045