feat(Request): Make list param parsing explicity

The idea is not to have to pay for what you don't use or want. Also
makes this consistent with req.get_param_as_int.

BREAKING CHANGE: get_param will no longer return lists for
comma-separated parameter values. Use get_param_as_list instead.

Implements #18
Merge pull request #63 from kgriffs/master

Issue #18