Miscellaneous fixups

Hi there!

A few fixups: docs, parameter validation, and wrapping open(...).read() in with statements. I've tried to separate these enough to let you keep just the ones you like.

c14cf57 creates a single point of truth for parameters to /post, /post/edit, and /post/reblog. This should let you revert 5e08b7523aa85453016389c06c337528f42f6b80, which disabled parameter validation in TumblrRestClient.edit_post.

All tests pass.

Cheers,
Marti