Gocept.collmex

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- Added testing helper ``create_activity``.

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- Added API for retrieving activities (``get_activities``).

- Updated tests and test infrastructure to recent changes in Collmex.

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- Added API for retrieving projects and creation of activities.

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- Make models robust against API changes so they don't immediately break when
the record becomes longer.
- Updated customer model to current API.

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- Fixed multi-threading bug: thread-local data needs to be intialized for each
thread.

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- Values returned from Collmex are converted to unicode.
- Cache results for the duration of the transaction.