Added sorting to project issues page

Adds functionality to sort issues on project pages by:
* Newest
* Oldest
* Recently updated
* Last updated
* Milestone due soon
* Milestone due later

![screen shot 2013-11-22 at 1 17 44 pm](https://f.cloud.github.com/assets/1238532/1603052/a541f674-53a2-11e3-9290-f11b5e092897.png)

This feature has been requested here http://feedback.gitlab.com/forums/176466-general/suggestions/4150983-it-would-be-helpful-to-sort-issues-by-their-associ and here http://feedback.gitlab.com/forums/176466-general/suggestions/3935995-issues-should-be-sorted-by-different-columns. This functionality would help our team discuss and work on issues by easily sorting them by different priorities.

I achieved this by adding a sort method to the existing `Issues::ListContext` class which is called in 'Issues::ListContext#execute`.