Added STRIP_NULLS as a URI constant.

When included as a flag on a representation factory, this should
indicate a desire for a representation writer to strip out any
null values from the output, rather than rendering the null field
explicitly.
[maven-release-plugin] prepare release halbuilder-api-2.1.1
[maven-release-plugin] prepare for next development iteration
Merge branch 'release/2.1.1'

* release/2.1.1:
  [maven-release-plugin] prepare for next development iteration
  [maven-release-plugin] prepare release halbuilder-api-2.1.1
  Added STRIP_NULLS as a URI constant.
  Added license
  Corrected parameter names - href instead of url, rel comes first.
Merge tag '2.1.1' into develop

Rel

* tag '2.1.1':
  [maven-release-plugin] prepare for next development iteration
  [maven-release-plugin] prepare release halbuilder-api-2.1.1