Rename upload/download methods to clarify directionality.

- 'set_contents*' -> 'upload'.

- 'get_contents* -> 'download'.

Original boto-like names retained as aliases per @jgeewax.

Fixes #148.
Merge pull request #320 from tseaver/148-rename_key_upload_download_methods

Rename upload/download methods to clarify directionality.