Adds 'ember:custom' include to portkey.json.

- This is needed to pull in the ember extension's code.
- 'custom' does not bring in file patterns from portkey.json
  but includes syntax highlighting and the `cname` and `mname`
  filters for use in templates.
- without this only the core portkey feature set would be available.
Changes :Rfoo to :Efoo.

- The resource commands are of the form :Efoo.
  Eg:- :Emodel, :Smodel, :Tmodel, :Vmodel and :Dmodel
- :R goes to the related file for the current file
  and :R! creates it if absent.
Merge pull request #4 from dsawardekar/feature/minor-changes-to-portkey-and-readme

Feature/minor changes to portkey and readme