[agnoster-theme] modifying theme for dirty/staged files

Changes to show dot for dirty files and plus for staged files in git repos

e.g.:
![Screenshot_1_30_13_8_49_PM](https://f.cloud.github.com/assets/1200246/113266/7ea7f9d4-6b48-11e2-89ea-914c7db48ca8.png)