Post meta handler; Allow for updating, adding, and deleting of post meta. Filter for passing sanitization callbacks; filter for requiring santizition callbacks. This method assumes values are passed as they are intended to be stored (array or single). In response to #68

Remove sanitization callback functionality - see #68
Pulled in post_meta progress from tlovett1 in #167.
Added methods to verify post meta should be updated or added.
Block serialized meta fields from read/write
Clarify missing meta key code
Use a switch for post meta actions
Ensure our meta output matches the expected input
Ensure data is slashed
Use meta ID instead of meta key
Allow adding multiple post meta values
Switch to hybrid approach
Only check for error when actually using meta
Update meta inline docs
Allow blank key or value
Don't require meta ID for prepare_meta

The ID should be in the meta_id field anyway.
Use prepare_meta in get_all_meta
Abstract edit permission check
Don't double-check in handle_post_meta_action

These cases are handled by the underlying add_meta/update_meta, so
there's no need for them here.
Trim some excess whitespace
Allow updating just the meta key too