Add a javascript runtime gem

The command 'bundle rake install' fails after bundling due to missing
javascript runtime, which is usually included in the Gemfile by default
in a new Rails 4 project. Added therubyracer gem to Gemfile, along with
standard comment with link to additional information.
Change label on new admin view to username

The new admin screen that comes up initially upon booting the app labels
the first field 'Name', but the subsequent login form asks for 'Username'.
This was a point of confusion for me. Changed the label on the new admin
view to reference the same label translation as the login form.
Merge pull request #40 from Yardboy/master

Add a javascript runtime gem, change label on new admin view to match login form