Providing get_module_path()

Hi @example42,

I'm prepping a PR for another module of yours. For this module, I require to have the absolute path to the module's directory (I'm implementing this strategy: http://jsosic.wordpress.com/2012/12/04/managing-ssh-host-keys-with-puppet/ , which uses generate()). 

The puppetlabs stdlib provides this functionality, but I can imagine that you don't want to add an extra dependency. Therefore I copied the function from puppetlabs, and put it in this PR.