Introduce new API - fully compatible with stdlib

*default behavior of fastcache is changed to raise TypeError on
 unhashable arguments to be 100% consistent with stdlib.
*introduce a new argument 'unhashable' which can take the values
 'exception' (default), 'warn', and 'ignore' to modify the behavior
 of fastcache upon receiving unhashable arguments.
*Altered docstrings to highlight this
 
addresses #2