improved Graph().parse()

[nicholascar]

Proposed Changes

• raises a ParseError, not a mysterious XML SAX error, when failing to parse in graph().parse()
• default parse format is now Turtle, not RDF/XML
• understands format "xml", i.e. file.xml, is RDF/XML
• includes tests for passing & failing to parse
• updates all tests that relied on parse() default being XML to use format="xml" - they all now work
• removes pointless Graph().load() method since it just called parse() with bad defaults

[coveralls]

Coverage increased (+0.08%) to 75.725% when pulling 3afffcd on improve_graph_parse into 9429538 on master.

[ashleysommer]

Will add back in the graph.load() function but it will create a deprecation warning.