Feature: Cross-site task timer

[saimonmoore]

This code adds the following feature to teambox (desktop only currently):

• On the task page, click to open "add hours" panel. Clicking on the little play button will open a div in the header of the page.
• You can navigate around the site and the timer keeps on ticking.
• Within the timer panel, you can then update the number of hours for that task and additionally close the task with/without a message.
• Before updating the task you can optionally start the timer ticking again or cancel it completely.
• The timer survives a browser close.

TODO:

• Make it look decent. My css skills are lacking so I'm hoping someone else can style this feature right.
• Mobile is currently not supported due to the fact that this solution is a pure js solution (cookies) rather than server-side. Wouldn't make much sense on mobile unless the timer was maintained server-side.

[saimonmoore]

Replaces: http://github.com/teambox/teambox/pull/2

Has additional commit to show task title and also continue/cancel timer once stopped.