BuiltIn.Log pprint support

This resolves issue #1666, Use Python pprint module inside Log keywords.

I've modified Log to use pprint.pformat internally.

Tested in OS X 10.9.3 using Python 2.7.5 and Jython 2.5.3.
Is this correct? If given object is string, its repr is written to stream, and otherwise the pprinted value is returned.