Add mockable functionality

The basis of the mockable module is to be able to determine whether the mockable version of given functions should be used or not.

Examples:

mockable_set('*') make the system use mock objects whenever possible
mockable_set('time_*) make the system use mock function for all functions beginning with time_
mockable_reset('*') make the system use real data everywhere (the default)
mockable_reset('time_*') make the system use real data for time_* and mock functions for anything that has been previously set.

This should be tested.