Add file for Deployment class

ticket:2
Basic Deployment class

ticket: 2
First pass at #210
Add GitHub#temporary_basic_auth context manager

- Included: Tests!
Use new context manager
first pass at tests, uncertain about this as well
Fix removing application auth tokens
Encode all generated error messages
Add decorator for new methods requiring app creds
Apply decorator to appropriate functions
Add GitHubSession#retrieve_client_credentials

- Convenience method for revoking authorizations
Fix sketch of requires_app_credentials
Rework revoke_authorization(s) with new methods
Doc-strings for tests

- ALso remove the unnecessary pieces of the tests
Merge branch 'pr/213' into develop
Allow for a callback to be specified

- Closes #212
List deployments

- TODO: Find a repo to test against
Add ability to create a deployment
Use proper headers
Fix construction of iter_deployments iterator