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'd say the more documentation the better. I do realize it'd be a lot to ask for us to go through and do this for every method, but I see no reason not to take advantage of more thorough documentation that already exists.

Also, consider that these docstrings directly create our documentation via Sphinx, so users will likely be looking for tutorials.