Added relativeTo to FileSystem type provider

Adds a second static parameter "relativeTo" to the FileSystem type
provider to enable building relative paths from any FileSystem location
Merge in other changes and correct tests

Corrected tests to handle new (proper) relative path formats.
Extended relative path testing

Added test for relative path to itself
Changed to platform neutral relative paths

The previous mechanism of generating relative paths required Windows API
usage, which would fail on non Windows platforms.  Switched to all
managed approach.
Merge pull request #21 from ReedCopsey/master

Implements Issue #20