Merge branch 'rel-0.7.0' into stable

Release 0.7.0 with changes as specified in changelog
Merge branch 'stable'

Merge stable back to master
Changed over to requests from httplib2
Added requests library to requirements.txt
Fixed requirements generated by pip freeze as per https://bitbucket.org/dhellmann/virtualenvwrapper/issue/218/mkvirtualenv-creates-env-with-2
Fiddled to function to use variable named body to satisfy tests.
Modified unit tests to allow for changed function calls. Fixed stray typo in api.py
Changed reference from httplib2 to requests in setup.py
Further fixes to tests/api.py interaction.
Enable correlation id to be passed in when creating a payment
Add getting refresh token and exchanging it for access token feature to api. Enhance headers method to allow passing in of refresh token. Post now takes refresh token token as well. Json dump to string moved to request from post
PEP8 compliance
pep8 compliance
PEP8 compliance
remove capture_refunds.test
Enable population of header when correlation id is passed into payment.create()
Modify mock tests to allow for refresh token parameter
Add sample for future payment
Remove unused imports
Add unit test for payments module for creating a future payment