Add support for passing content type via the options

This needs to be done to support the _session api which requires
application/x-www-form-urlencoded; charset=utf-8
A way to pass a session cookie to the request, via opt.Cookie
Merge pull request #4 from ryanramage/header_opts

Add support for passing content type via the options
Merge pull request #5 from ryanramage/cookie_opt

A way to pass a session cookie to the request, via opt.Cookie
bump version
turn off caching by default
bump version