Support helmet.defaults() as middleware, fixes #13

Here's a simple way to support both the old and more conventional middleware-like usage of `helmet.defaults()`.

NOTE: there's one caveat in that the user *must* be using (i.e. have installed) either 'express' or 'connect' themselves but that seemed a reasonable enough assumption, eh?
Fix defaults-only middleware case, i.e. no args
Merge pull request #29 from natevw/patch-1

Support helmet.defaults() as middleware, fixes #13