Allow overriding build_type.

This is useful for testing the various publishing tasks on branches
other than master/beta/stable.
Merge pull request #74 from rjackson/allow_overriding_build_type

Allow overriding build_type.
Make static test site.

Running the following from the ember repo should result in a static file
to run the test suite on (make sure to run `rake dist` first):

Rake task:

```
rake ember:generate_static_test_site
```

Manually:

```ruby
require 'bundler/setup'
require 'ember-dev'

generator = EmberDev::TestSiteGenerator.new(static: true)
generator.save('dist/tests.html')
```