Refactoring IFileSystem

Making AssemblyNeutral and creating .Interfaces package
Changing semantics of string property to be relative
Updating tests
Changing return value to be always non-null with Exists property
Yes - Only relative paths will be supported.

With the previous code these were supported:
1. Absolute paths rooted at application folder
2. Relative paths with a leading slash.
3. Relative paths without a leading slash. 

With the new changes (1) will not be supported any more. This code still has some bugs that I will be fixing and updating the PR.