Fixes Bug #114: New-Fixture raises exception when using $pwd directly as...

When creating a New-Fixture and using $pwd or $(pwd) as -Path parameter, the powershell will throw an Exception caused by powershells dynamic typing.

````
New-Fixture -path $Pwd -name "Out-Log"
Method invocation failed because [System.Management.Automation.PathInfo] doesn't contain a method named 'TrimStart'.
````
This change restricts parameters to type [string] instead of letting powershell decide which datatype to use.