Fixes for Mono-3

* revise string extension to work properly in Mono 3 (and faster for
  Windows/.Net and Mono 2)
* fix project file to work around bug in Mono 3 xbuild
* fix/enhance audio test to avoid SIGSEGV under Mono 3

These changes are compatible with Mono 2 and Windows/.Net.
Fix audio tests to ensure playing stops before the file is deleted
Protect string extension method against concurrency
Merge pull request #44 from sillsdev/FixForMono3

Fixes for Mono-3