Remove Graph.add(Collection), fixes #11
Change Iterator to Stream to take advantage of Java-8

fixes #10

Also removes booleans from add/remove/clear methods to make it possible
to asynchronously implement these methods, possibly in parallel, without
synchronisation. Having to return a boolean requires that the method be
implemented synchronously, reduces parallelism, and requires
synchronisation at some point to ensure the return value is accurate.
Add Graph.contains methods
Add null wildcard information to Graph.contains javadoc
Add javadoc for the Graph.contains method return values
Merge pull request #12 from ansell/graph-stream

Remove Graph.add(Collection), add Stream, remove boolean results from add/remove/clear