IPAddress and IPAddressWithMask updates and fixes

* Provide asCidrPrefixMask to determine if an IPAddress represents a valid CIDR netmask
* cidr prefix length computation was broken:
  + 0 not detected as valid CIDR
  + BigInteger is signed. Grrr
* Add more unit test cases
* Test asCidrPrefixMask
Bump openflowj-loxi artifact version
Add null pointer checks and associated test cases.
Address review comments
Merge remote-tracking branch 'floodlight/master'
Bump version number for openflowj-loxi maven artifact
More test coverage
More review comments. Fix concurrency issue.
Merge into master from pull request #155:
IPAddress and IPAddressWithMask updates and fixes (https://github.com/floodlight/loxigen/pull/155)