Fix for issue #19 - posh-hg from nuget console

There were two small issues not allowing posh-hg to work from inside Visual Studio's package manager powershell console. 

* The first relates to PowerTab and Register-TabExpansion, according to fixes in posh-git the NuGet console has its own flavour of this method, I applied the exact same fix as done in posh-git inside the HgTabExpansion file.
* The second fix pertains directly to issue #19. The problem is that the NuGet powershell console returns -1 as background colors in the hg settings, crashing the existing Write-HgStatus. When Write-Status does not return PowerShell appends the default prompt. I made a similar change as done in posh-git, all write-host methods in Write-HgStatus were changed to go through a new intermediate function write-prompt that checks for a -1 background color and if it finds one ignores the background color.

Below is a screenshot of posh-hg working in Visual Studio 2012, I have checked it works in Visual Studio 2013 Preview as well.
![image](https://f.cloud.github.com/assets/254663/756702/e97696ba-e638-11e2-9c84-3768ebaeac7e.png)