Put cache database in user-writable location

[MCGallaspy]

@aronasorman @benjaoming let me know if this seems tenable. Will test it out on the deb by pulling source and setting IS_SOURCE=False, assuming this essentially replicates installing using deb.

[MCGallaspy]

For posterity, the problem was that after installing using a .deb on raspberry pi, the running user didn't have sufficient permissions to write to the affected database.

[benjaoming]

Looks good! Testing it is a matter of installing it to a real virtualenv rather than using pip editable mode (-e)

workon YOUR_VIRTENV
python setup.py sdist
pip install dist/ka-lite-0.15.0.tar.gz
cd somewhere/else
kalite start --foreground

[MCGallaspy]

This seems to work as expected. Recommend merge.