Add methods generating & testing uniqueness of IDs

Add methods:
* self.generate_id
* self.integer_time_hash
* id_persisted?
* uri_persisted?
Add generate_id tests; Modify generate_id params

Change params for generate_id to be a single parameter hash. Add parameter validation.
Merge branch 'master' into generate_id to include latest pushes to master
Add IDMinter class taking lambda function

Per recommendations from Tom Johnson, the following changes were made to this branch...
* remove generate_id method from Resource class
* add new IDMinter class with generate_id method
* change generate_id to take a lambda function for the minter; include UUID generator as the default minter
* update Resource.id_persisted? and uri_persisted? to call method in Repositories class
* add method has_subject? to Repository class which uses RDF::Repository.has_subject? to check for the existence of an rdf_subject in registered repositories.
** can specifiy a single repository to check.
** will check all registered repositories by default.
NOTE: Merged with latest master before updating branch in github.
change minter to take a block as last parameter; change from id to local_name
merge with master
Merge remote-tracking branch 'github/master' into generate_id
Remove local name minter

The code providing local name minting is moving to a separate gem.
Merge pull request #61 from ActiveTriples/generate_id

Add methods generating & testing uniqueness of IDs