Merge pull request #5 from dead10ck/master

Split Graph and DiGraph
Merge pull request #6 from dead10ck/master

New examples + docs; IDs can be any hashable type
Merge pull request #7 from dead10ck/master

Caching
Merge pull request #8 from dead10ck/master

Operators, algorithms, VCDB example, ASN Timeline example
Merge pull request #9 from dead10ck/master

Add `get_or_add_node()`. Graph attributes. Added wiki.
Update repo link in installation instructions
Use setup.py installation
Remove unnecessary section in README