[bugfix] Make martini support standard bugsnag API
[bugfix] Make martini support standard bugsnag API

This commit ensures that a project that is only set up using
`bugsnagmartini.AutoNotify` will get all the features from the standard
bugsnag package as well, in particular session tracking.

bugsnag/bugsnag-go,bugsnag/bugsnag-go,bugsnag/bugsnag-go