Add support for extensionless files, other improvements

* If a file has no extension, a HEAD request is performed to see if it
* exists, and if not, a `.js` is added to the url (Fixes GH-2)
* Support cases like `require('foo/bar')`, going to the npm module `foo`
* Ensure valid input
Merge pull request #3 from remixz/file-path-improvements

Add support for extensionless files, other improvements