Improve displaying status using gitprompt tool.

Change description:
  1. When doing Get-GitStatus, run gitpromptclient.exe instead of running git.exe.
  2. GitPromptClient talks to GitPromptCache.exe to retrive the cached result (It may take same amount of time as 'git status' for first time). 
  3, During install, posh-git will download the binaries and store it under bin directory. 

Gitprompt tool is available here    https://github.com/fieryorc/gitprompt

Advantages:
 1. The cache is maintained in a seperate process, and it serves as a cache for multiple powershell instances.
 2. It is C++ process using libgit2, so it is lean and mean, also very fast.