Change issue tracker link to point to the new one.

From now on, it's https://github.com/ariya/phantomjs/issues.

For the details, check this mailing-list discussion:
https://groups.google.com/d/msg/phantomjs/It04OsP7csU/Vw2cmJkAILUJ
New "page.loading" and "page.loadingProgress".

Now you can know about the Page Loading Progress
using 2 property:
- [bool] page.loading
- [int: 0-100] page.loadingProgress

Implements #1091: http://code.google.com/p/phantomjs/issues/detail?id=1091
Tests for the new "page.loading" property.