Fix PANDOC_PATH issues

I've added a module level method that sets the PANDOC_PATH as using the `PANDOC_PATH = 'blah'` method wasn't working as described in #5. I've also set the default PANDOC_PATH to just `pandoc` as discussed in #3

*Duplicate of @yoavram's initial [pull request](https://github.com/kennethreitz/pyandoc/pull/7) as there was an overlapping change. Merging to his branch to reduce work on @kennethreitz*