Select Template Compiler on Use

Users of Ember-CLI were having trouble setting
`Ember.ENV.I18N_COMPILE_WITHOUT_HANDLEBARS` before Ember-I18n
loads. This commit changes the default `Ember.I18n.compile` to
select its implementation lazily on first use so the users'
settings have had a chance to take effect.

Closes #139
Merge pull request #140 from jamesarosen/delay-select-compiler

Select Template Compiler on Use