[#32067] Added custom logo and background color gradient for login area in isis template

Before:
![before](https://f.cloud.github.com/assets/441774/1186879/ee4cc056-232d-11e3-8e26-427227ee84df.png)

After:
![after](https://f.cloud.github.com/assets/441774/1186878/ee0e424a-232d-11e3-9d5c-af2d82c1f59f.png)

Tracker: #32067
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32067&start=0