Fix printing non-existent error

This caused a segfault when unreffing a camera object which was still recording
but stopping it was not failing.
Add Python support code to release GIL on grab

A compile time switch WITH_PYTHON_MULTITHREADING has been added that toggles GIL
releasing.
Merge pull request #60 from ufo-kit/release-gil

Release GIL
Pick release notes of version 1.6.1
Fix #61 causing a race condition

The buffer thread now waits on the *intent* to stop the recording (i.e. the
internal state) instead of using the external state information.