Fix #2, Preserve environment boolean.

* This provides the capability to preserve the environment for the child
  process from that which is passed in to Sprok itself.
* The tests are kind of brittle (checking some shell command?). As far
  as I can tell, the shell command is not used by anything bar tests.
Merge pull request #3 from fujin/add-preserve-environment

Fix #2, Preserve environment boolean.

We can address the TODOs and duplicate keys in follow-up commits.