refactored file-or-str opening. fixes #191

[bmcfee]

This PR implements #191, and is mainly lifted (and simplified) from a similar helper in JAMS.

Changes:

• A new (private) helper function _open consolidates repeated code for handling string-or-file-like inputs in IO.
• The new function uses contextlib to properly handle file descriptor cleanup.
• The exception type for load failure was changed from ValueError to IOError; tests were updated accordingly

[craffel]

Awesome, thank you. I assume this is tried and true since it appeared in JAMS first. The only pedantic comment I have is that _open is not a very descriptive function name, I would call it something like open_file_or_str (or _open_file_or_str if you insist on the pseudo-private function/not supplying a typical docstring). Change that if you want, then feel free to squash and merge!