add post() method

This will (in a later commit) be used to implement logins. It's a little
ironic that with my last commit having removed urllib usage, I'm now
re-importing it to get the urlencode() function.
Make key parameter of load() optional.

Some of the strava API calls return multiple pieces of useful data, such as
the /v2/authentication/login call. Rather than discard that data having the
key optional means you can have the load() method return it all to your
method.