Added *_factory arguments to Fypp ctor, to allow custom components

[rhdtownsend]

These small changes allow customization of the parser, builder & renderer components -- in my particular case, to allow automated dependency generation to work. One possible weakness is the use of inspect.signature to check the signature of the factory methods -- not sure this will work for python < 3.3.

[rhdtownsend]

Added evalauator_factory argument, together with docstring entries for new arguments and Change Log entry.