(SERVER-61) improvements to the ExecutionStub

* The execution stub now returns an instance of
  Puppet::Util::Execution::ProcessOutput, as per the contract of
  Puppet::Util::Execution.execute.

* Add basic spec/unit tests

* In java, close the reader used to read the process's STDOUT.

* In ruby, close the file handles passed-in for STDIN, STDOUT, and
  STDERR; we're not using these (this is implemented in java instead),
  but Puppet::Util::Execution.execute does not close them when an
  execution stub is installed.
I think not.  The contract of `P::U::Execution.execute` is pretty specific about that type of error:

https://github.com/puppetlabs/puppet/blob/master/lib/puppet/util/execution.rb#L131

Given that, I think it's inappropriate to use `P::ExecutionFailure` here.