Support gradle task cache

[tnielens-centrica]

The tasks of this plugin don't seem to support the gradle task cache.

• Expected behavior
  When I configure my jaxb generation to strip varying parts of the generation output with args = ["-npa", "-no-header"], I expect the gradle cache to kick in, and the output of the generation to be retrieved from the gradle task cache.
• Observed behavior
  The output of the generation is not retrieved from the task cache. The task isn't configured for caching.
  From the log:

Caching disabled for task ':my-project:jaxbJavaGenMyName' because:
  Caching has not been enabled for the task

Relates to #70