Fix missing commits bug in the default graph database.

Makes TinkerGraphDatabase.Impl implement BlueprintsGraphDatabase.commit()
so that the graph is saved to disk on each update. Otherwise all the
changes were only in memory until TinkerGraphDatabase.close() was
properly called, which frequently leads to missing entries in the graph
database.

A test case was added to TinkerGraphDatabaseTest, and the abstract
test suites for graph databases were improved so that they don't require
a full repository to be run.
Merge pull request #630 from groldan/graphdb_persistence

Fix missing commits bug in the default graph database.
Ignore tags when cloning

tags are now bypassed when cloning. This is a workaround so repos with tags can be cloned (before it tried to treat all refs as branches and it failed whan a tag was found), but the clone operation is still uncomplete and tags will not be cloned
Merge pull request #636 from volaya/fix_clone_tags

Ignore tags when cloning