Added method to close RexsterClient connections.

See discussion toward the end of inthefabric/RexConnect#7 for details.
replacing RexsterClientFactory with getClient
working on the json rexpro serializer
fixing json converter
fixing json deserialization of null values
adding method to indicate primitive keys are allowed
making some changes to help with jackson's json number serialization
Merge branch 'rexclient-close-conn' of github.com:zachkinstner/rexster into pull-308
fixing session request array size
Merge branch 'rexpro-v1' into pull-308