CBLModel now supports remaining scalar (int/float) property types.

Added support for 64-bit integers and 32-bit floats.
Added unit tests for all scalar types.
Thanks to Žymantas Paulauskas for the 64-bit support!
Clear cache and save models when database is closed

Clearing the cache breaks a ref cycle that caused the db to stay in
memory until its doc cache got purged by the OS, at which time a
deadlock was possible. (Fixes #164.)