Simplifies testem configuration to reduce duplication

Instead of specifying the source and serve files, this change
utilizes the test_page option to test index.html directly and load
all the scripts as specified in that file.

  - Removes src and serve options from grunt testem task and from
    testem.json
  - Adds testem.js to index.html as per
    [testem docs](https://github.com/airportyh/testem#include-snippet)
  - Adds routes to testem configuration so that testem can correctly
    locate test files and assets as referenced in index.html
Merge pull request #400 from astonishdesign/feature/simplified_testem

Simplifies testem configuration to reduce duplication
[#401] Fakes testem.js so that in-browser tests run

Testem requires a script tag to load /testem.js, which is supplied
by Testem. This file doesn't actually exist on the file system, which
causes localhost:8000/tests to fail with a global failure.

By telling express to serve up a fake testem.js file, the script load
is satisfied, thus making it posible to execute the tests in both
the Testem and direct contexts.
Merge pull request #402 from astonishdesign/feature/fix_inbrowser_tests

[#401] Fakes testem.js so that in-browser tests run
upgrade to latest es6-module-transpiler
a sane default RSVP onerror now exists in ember