Issue title: warning: no files found matching 'README.md'
 Issue body: Small issue, there is a warning on setup, maybe MANIFEST.in is not including README.md in the distribution (maybe because MANIFEST is failing to include README.rst in its first line.


```
  Running setup.py install for pyshorteners
    Skipping installation of /home/rochacbruno/.virtualenvs/quokka/lib/python2.7/site-packages/pyshorteners/__init__.py (namespace package)
    
    warning: no files found matching 'README.md'
    Installing /home/rochacbruno/.virtualenvs/quokka/lib/python2.7/site-packages/pyshorteners-0.4.1-py2.7-nspkg.pth
```
 Comments: 
 Comment 0: https://github.com/ellisonleao/pyshorteners/blob/master/MANIFEST.in#L1
 Comment 1: Forgot about the Manifest! Thanks @rochacbruno . Did you try to install the current master branch, right?
 Comment 2: Fixed on https://github.com/ellisonleao/pyshorteners/commit/617834f466c418b94fb2255c0f8020005c600806

md file added. New version `0.4.2` released
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