Update Path.cs

Fixed Path.GetDirectoryName() bug when path is in the following form: 'C:filename'
Added tests to PathTest.cs
When doing a compile time check for large file support there is no need for wrapping in a main function.  In fact is can cause failures in some cases
Merge pull request #667 from TalAloni/mono-2-10

Mono 2 10 - Path.cs Update
Merge pull request #673 from symform/symform_lfs_check_no_need_for_main

When doing a compile time check for large file support there is no need ...
Atom10FeedFormatter should generate subtitle tag for Description property
Merge pull request #699 from linquize/atom-bug-2-10

Atom bug 2 10