Adding custom handling capability for session load errors
Version bump since adding functionality
Fast-failing when handlers are not callable
Changes per feedback on #24
Tweaks to specs after rebasing on top of #16
Always destroying sessions when load fails

plus consolidation of various destroying methods
Updating docs for latest on_session_load_error signature
Merge pull request #24 from modcloth/marshal-load-handling

Adding custom handling capability for session load errors