Move openapproved into the reviewstats namespace.

This involves updating one of the shell scripts which now depends on
'pip install -e .' or 'pip install reviewstats' (for a release) having
been run.

Change-Id: I2e63501c4a41ed0b20b1ab7ba1e2f78beed04962
Move reviews_for_bugs into reviewstats.

Change-Id: I4584b1fed09b0e10f144b9110457436dd04d601d
Move openreviews into the reviewstats namespace.

Change-Id: I3029a4568c74b227412b677fcb5e6df6c040458f
Move reviewers.py into the reviewstats namespace.

Change-Id: I962cd777cb3507d26fd146f553236f7c6e9af772