test_pytumblr.py: ensure tumblr_credentials.json gets closed

Put the read in a with statement to close the file when we're done
with it.
TumblrRestClient.send_api_request: ensure files get closed
TumblrRestClient.create_text: reject invalid 'text' kwarg
TumblrRestClient.reblog: add 'comment' param to docstring
TumblrRestClient: miscellaneous parameter validation fixups

Add a _post_valid_options() method, which returns a list of valid
options for a given post type (which may be None, in which case the
common list of valid post options are returned). This lets us remove
the valid_options argument from _send_post() and fix up the parameter
validation in the reblog() and edit_post() methods.

While we're here, also add missing valid parameters to the edit_post
docstring.
Merge pull request #37 from mbolivar/fixups

Miscellaneous fixups