Make `Redmine` class picklable

The `Redmine` class couldn't be pickled because the `gettatr` would attempt to
turn any attribute access into a `Resource`, even `__getstate__`.

The proposed fix treats attributes that begin with an underscore (and
double-underscore) as non-`Resource` attributes.

Fixes #64
Merge pull request #65 from rconradharris/make-redmine-picklable

Make `Redmine` class picklable