Added option to retrieve Fitbit profile measurements in English units.

By default, Fitbit responds to api-get-user-info (profile) calls with values for height and weight in metric units. This change allows a developer to request English units. Example:

```ruby
Rails.application.config.middleware.use OmniAuth::Builder do
  provider :fitbit, 'consumer', 'secret', { :use_english_measure => 'true' }
end
```