Feature descriptor optional and check consistency

These commits refactor ConfigurationParser.getRepositories and add two additional repository options:  `descriptorOptional` and `skipConsistencyCheck`.  These correspond to the Ivy API calls `r.setDescriptor(BasicResolver.DESCRIPTOR_OPTIONAL)` and `r.setCheckconsistency(false)`, where `r` is the resolver in question.  The modifications pass all of the tests in the sbt test task and a couple of additional tests added to exercise the additional flexibility available in `ConfigurationParser`.