increment sqla model before commit to avoid session trouble.
Workaround bug in webob.Response.from_file

Convert Response headers to str instances instead of unicode since it is
what WSGI expects.
Add an after_commit hook on ext.sqlalchemy using identities rather than entity instances
Merge pull request #10 from deuxpi/webob-str-headers

Workaround bug in webob.Response.from_file
Style
Implement master switch to globally disable caching
Improve tests for redis key_versioner
Move key_versioner / redis master_version check into separate method
Handle condition where the master_version is set successfully but is still unavailable