Filtering categories in collections

- added method to get all categories related to a given category from a collection
- improved get method in model to use new filtering method of collection
- added and updated methods in client to allow retrieval of all entity, resource and link types and type identifiers
- added and updated rspec tests for collection and model
Category filtering for OCCI binary

- added option -F and --filter to occi binary options
- added category filtering to occi binary when dumping model
Parser

- fixed parsing of links if there is only one resource present in the collection
- removed the default for the rel attribute in links as this may lead to irritations
Merge pull request #36 from ffeldhaus/bugfix-json-link-parsing

Parser fixes
Merge branch 'feature-filtering' of https://github.com/ffeldhaus/rOCCI
Merge pull request #35 from ffeldhaus/feature-filtering

Filtering categories in collections
Merge remote-tracking branch 'upstream/master'