More helper methods on PathRepresentation(s)

[lsymds]

Add some more helper methods/properties to path representations.

• Retrieving a file's extension.
• Retrieving a file's name, less the extension.
• Comparison operators between paths.
• Path at index (i.e. a/b/c with PathAtIndex(1) would return 'b').
• Contains directory
• Is in directory

[lsymds]

https://github.com/getbaseline/Baseline.Filesystem/tree/issue-34

[lsymds]

Don't need as many of these; just the comparisons and methods related to extensions will do.