scrolling controls

This change adds two scrolling related features:

1) Key commands can now scroll the output of the selected job: PAGE-UP/DOWN, HOME, END (optionally with ALT-).  ALT-SPACE/SHIFT-ALT-SPACE  act like PAGE DOWN/UP. These work even if the command line is focused.

2) Output from commands is no longer scrolled, unless the last output has been visibile for a period of time.

The net effect of these two features is that commands that dump a lot of output, such as 'cat README' are effectively run though a pager: The first screen full will be visible, and pressing ALT-SPACE or PAGE-DOWN will page through the output. Commands that output over time, such as 'tail -f' will scroll to show successive lines, so long as they don't come to fast.

commit: http://code.google.com/p/plush/source/detail?r=cfac0ba5a634f07150b9c501ed078cd126f3f1d9

[mzero, 2012-08-01 01:49:05]

please review

[mzero, 2012-08-01 02:39:26]

please review

[Jasvir, 2012-08-08 20:42:36]

LGTM

http://codereview.appspot.com/6452074/diff/1/static/js/jobs.js
File static/js/jobs.js (right):

http://codereview.appspot.com/6452074/diff/1/static/js/jobs.js#newcode80
static/js/jobs.js:80: scrollIntoView(scrollback, currentTopic);
Fine as is, but fyi closure compile will complain here since your function is
declared to use 3 args but you explicitly only used 2.