Allow for wheel package creation

addresses issue #93

FWIW, I'm not sure if wheel should exist in requirements.txt, as the average person won't need it. So makes me wonder if there shouldn't be a dev-requirements.txt and a requirements.txt with dev-requirements.txt including requirements.txt? That way you can install dev stuff or just the necessary stuff for github3.py. 

Also, I couldn't test uploading the wheel, but it does create it: 

```
ls -l dist
total 200
-rw-rw-r-- 1 bmorriso bmorriso 115958 Sep  6 21:34 github3.py-0.7.0-py2.py3-none-any.whl
-rw-rw-r-- 1 bmorriso bmorriso  85349 Sep  6 21:35 github3.py-0.7.0.tar.gz
```

and it does successfully install

```
pip install --no-index dist/github3.py-0.7.0.tar.gz         
Ignoring indexes: https://pypi.python.org/simple/
Unpacking ./dist/github3.py-0.7.0.tar.gz
  Running setup.py egg_info for package from file:///home/bmorriso/localrepository/github3.py/dist/github3.py-0.7.0.tar.gz
    
    no previously-included directories found matching '*.pyc'
    warning: manifest_maker: MANIFEST.in, line 7: 'recursive-include' expects <dir> <pattern1> <pattern2> ...
    
    warning: manifest_maker: MANIFEST.in, line 8: 'recursive-include' expects <dir> <pattern1> <pattern2> ...
    
    no previously-included directories found matching 'docs/_build'
Requirement already satisfied (use --upgrade to upgrade): requests>=1.2.3 in /home/bmorriso/.virtualenvs/github3.py/lib/python2.7/site-packages (from github3.py==0.7.0)
Requirement already satisfied (use --upgrade to upgrade): uritemplate.py>=0.2.0 in /home/bmorriso/.virtualenvs/github3.py/lib/python2.7/site-packages (from github3.py==0.7.0)
Installing collected packages: github3.py
  Running setup.py install for github3.py
    
    no previously-included directories found matching '*.pyc'
    warning: manifest_maker: MANIFEST.in, line 7: 'recursive-include' expects <dir> <pattern1> <pattern2> ...
    
    warning: manifest_maker: MANIFEST.in, line 8: 'recursive-include' expects <dir> <pattern1> <pattern2> ...
    
    no previously-included directories found matching 'docs/_build'
Successfully installed github3.py
```