Rename upload/download methods to clarify directionality.

- 'set_contents*' -> 'upload'.

- 'get_contents* -> 'download'.

Original boto-like names retained as aliases per @jgeewax.

Fixes #148.
I found the chatty bit redundant and un-useful.  If we actually want REPL examples in each APIs docstring, then we should add them (but all the imports / setup noise there actually obscure the one interesting line).