Added a possibility to display the remote branch and the number of commits you are ahead or behind

Added a possibility to display the remote branch and the number of commits you are ahead or behind.

The theme "strug" demonstrates this new funtionality. There are some new variables:

`ZSH_THEME_GIT_PROMPT_REMOTE_STATUS_DETAILED` : allows you to switch between the current and the new display option.
`ZSH_THEME_GIT_PROMPT_REMOTE_STATUS_PREFIX` : prefix for the remote status information
`ZSH_THEME_GIT_PROMPT_REMOTE_STATUS_SUFFIX` : suffix for the remote status information
`ZSH_THEME_GIT_PROMPT_AHEAD_REMOTE_COLOR` : color for displaying the ahead information (currently only used for the detailed mode)
`ZSH_THEME_GIT_PROMPT_BEHIND_REMOTE_COLOR` : color for displaying the behind information (currently only used for the detailed mode)


Demonstration of the new functionality: 

![git_remote_status_detailed_demo_ahead](https://f.cloud.github.com/assets/128227/42053/9897d29c-55f9-11e2-9a20-7822ebc9e124.png)
![git_remote_status_detailed_demo_ahead_and_behind](https://f.cloud.github.com/assets/128227/42057/a44e1132-55f9-11e2-84cb-ecd0b15c9b4e.png)
![git_remote_status_detailed_demo_behind](https://f.cloud.github.com/assets/128227/42058/a849b700-55f9-11e2-9daf-afa46b7b6277.png)