* The parameterization of "delimiter" was incomplete.
* safe_substitute's code for braced delimiters should only be executed
  when braced is not None.
* Invalid pattern group names now raise a ValueError.  Formerly, the
  convert code would fall off the end and improperly return None.

Beefed-up tests.

* Test delimiter override for all paths in substitute and safe_substitute.
* Alter unittest invocation to match other modules (now it itemizes the
  tests as they are run).