Set expires_at based on current time, not just an arbitrary timestamp. CC #112.
Add an example for how to refresh oauth 2 tokens with Google.
Fixed oauth2 workflow doc. A wrong argument name was being used in Web
Application Flow.

* Fixed doc of oauth2 workflow.
* Web Application Flow defined a variable called `client_secret`.
* We intended to use `client_secret`, but were accidentally calling it
`secret`. Changed `secret` to say `client_secret`.
Merge pull request #114 from akshar-raaj/master

Fixed oauth2 workflow doc. A wrong argument name was being used in Web Application Flow
When signature_type is body, set appropriate content-type header.

Fixes requests/requests-oauthlib#115
Merge pull request #117 from xnox/fix-content-type-with-body

When signature_method is body, set appropriate content-type header.
Depend on the pycrypto enabled oauthlib in setup.py.

requests-oauthlib requires a pycrypto-enabled oauthlib. This is enabled in requirements.txt, but not when setup.py is called directly.
Merge pull request #120 from negz/master

Depend on the pycrypto enabled oauthlib in setup.py.