Refine docs and add more tests for Plug.MIME

- Refined docs adding `## Examples` headers to all docs (in order to be
  consistent with the rest of the Plug and Elixir docs) and fixing some typos
  here and there
- Added some doctests as well as tests for the `path/1` function
Merge pull request #153 from whatyouhide/mime-docs-and-tests

Refine docs and add more tests for Plug.MIME