Support test settings in MSTest

Not sure I'm missing something but FAKE doesn't seem to support MSTest test settings out of the box. I need it for specifying test settings in VFPT.

I added:

 - `TestSettingsPath` which specifies paths to *.testsettings files
 - the use of `TestMetadataPath` in `buildMSTestArgs` (I did Find All References and found that this field hasn't been used anywhere :-) )