Make the New-Fixture function accepts "." (the current workding directory) as the $path parameter

In the past when we run the follow code, we will get errors.

PS C:\temp> New-Fixture . HelloWorld
Test-Path : Cannot bind argument to parameter 'Path' because it is an empty string.
At C:\Users\eyang\Documents\WindowsPowerShell\Modules\Pester\Pester.psm1:197 char:25
+     if (-not (Test-Path $path)) {
+                         ~~~~~
    + CategoryInfo          : InvalidData: (:) [Test-Path], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorEmptyStringNotAllowed,Microsoft.PowerShell.Commands.TestPathCommand

Creating => \HelloWorld.ps1
Creating => \HelloWorld.Tests.ps1
PS C:\temp>

Now the $path parameter accepts ".". 

PS C:\temp> New-Fixture . HelloWorld
Creating => C:\temp\HelloWorld.ps1
Creating => C:\temp\HelloWorld.Tests.ps1
PS C:\temp>
Merge pull request #86 from ogrishman/patch-2

Make the New-Fixture function accepts "." (the current workding director...