Apparently in some build environments (e.g. Windows) the "symlinked"
version is not always created. This should (hopefully) always find it.

Support for loading libsodium.dll on Windows

Fix issues for Windows MinGW

- configure fails because `sh` is not executed
- load fails due to no libsodium.so

This fix works with [RubyInstaller](http://rubyinstaller.org/).
Fix #6

Update libsodium 1.0.1

rubygems interprets a letter on the end of the name as --pre :(

JRuby support