Fix implementation of string extension for Windows
Merge pull request #49 from sillsdev/FixStringExtensionForWindows

Fix implementation of string extension for Windows
Add NormalizePath method

Add a method that normalizes a path by converting backslashes to
forward slashes. This is useful when a path gets read from a file
that gets shared between Windows and Linux - if the path contains
backslashes it can't be found on Linux.

Change-Id: I18a27c1d7cef649ca9e5bc1b6eecd812f3472752