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.
Yes, this is correct. pprint recursively traverses data structures, writing to stream along the way. See the pprint source for more details: http://svn.python.org/view/python/branches/release27-maint/Lib/pprint.py?view=markup