test: implement end-to-end tests

Add two basic end-to-end tests checking that the User model and
the authentication are correctly set up by the template.
test/end-to-end: implement caching of npm packages

Speed up subsequent runs of unit-tests by caching the result of
`npm install`.
test: Increase `npm install` timeout

Increase the timeout to 120s to prevent the end-to-end tests from
timeouting on the CI server.
Merge pull request #70 from strongloop/e2e-tests

test: implement end-to-end tests