support html tags in translations

[jelhan]

Adding support for html tags in translation as it was discussed in #97. It's disabled by default as it could be a security risk. Could be enabled by configuration:

var ENV = {
  APP: {
    defaultLocale: 'en',
    htmlLocales: true
  }
};

[jelhan]

I'm not sure if we have to escape values used in interpolation or pluralization. They could be entered by users and should therefore not be treated as safe. Not sure if Ember.String.fmt already does this. We should definitely investigate this before merging.

[jelhan]

Interpolation values are mapped through Handlebars.Utils.escapeExpression() if htmlLocales is enabled in last commit. Should be safe now.