New-Fixture polished

- First I moved the New-Fixture, Create-File to separate file in functions. When module is imported these functions are included automatically.
- I discarded Write-UsageForNewFixture. Usage can be looked up by Get-Help, so inline script help is the only place to update if interface changes.
- New-Fixture now outputs objects of the new files instead of writing to Host. If file already exists it writes to Warning.
- Interface of the function is kept the same, but when specifying only -Name parameter fixture is created in the local folder.
- Path accepts full, relative, UNC paths and "." for local directory.
- Polished help.
- Tweaked the default test as proposed in Issue 57.
- New-Fixture tests added.