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.
@tobert I think if you declare an Interface type which wraps around the syscall.Exec, that probably outta do it, then you can have a Syscaller (forgive me) type which implements the syscall.Exec, and a Nooper type which just spews the args to the testing.T logger or something.