Fix the escaping of the PATH

* Fixes "$PATH" so it'll work with spaces in it.
* Fixes added directories for the PATH, if it has characters that need
  to be quoted.
* Added test for add_pre_command

Closes: serverspec/serverspec#392
This adds prepend_path and append_path

Note: I couldn't figure out how to alias `path` to `prepend_path`, so
this isn't backwards compatible.  But at least it raises an error.