Fixed support for the default namespace
- PROV-JSON mappings now encode/decode the default prefix as per www.w3.org/Submission/prov-json/
- PROV-N export now generates the correct default namespace line
Return pdbundle when calling add_prov_bundle
When exporting a bundle as a document add namespaces from parent
Changed: Bundles cannot have their own namespace prefixes
Better handling of ids when generating PROV-JSON
Merge pull request #19 from trungdong/anon-id

Better handling of ids when generating PROV-JSON
Changed: Removed namespace declarations in example bundles
Fixed minor bugs:
- Stop outputting prefixes in a bundle's PROV-N
- Minor change in the logic of parsing attribute types
- Fixed bug in AnonymousIDGenerator
- Fixed bug when initialising the default namespace
Fixed: Stop outputting prefix in an bundle's PROV-JSON
Added: is_document() and is_bundle() for ProvBundle
Fixed: prov.persistence only saves namespaces at document level
Add new bundle's namespaces to parent document
Added hash functions for Literal and Namespace
Add get_flattened() function to ProvBundle

Puts all bundle records into root document and merges attributes.
Fixed: errors in graph generation for edges referring to missing nodes
Fix references to bundle records when flattening
Merge branch 'master' of https://github.com/trungdong/prov
Fixed: Test for flattened documents
Added: check if a node of a relation already drawn
Fix for flattening documents