Add explicit parameters to Invoke-MSBuild

I'd like to see a Target parameter for Invoke-MSBuild added as `[string[]]` so we don't need to pass it as a string in MsBuildParameters. 

I'd also like a Properties parameter of type `[hashtable]` so we could write things like this: 

``` powershell
Invoke-MSBuild -Target Clean,Build -Properties @{Configuration='Debug';OutDir='..\artifacts\'}
```
