Extract translations into separate files

This way if an application does not need a particular translation, the
file can just be omitted and the overall file size will be reduced.

Adding new translations becomes easier this way, too.

Many modern web frameworks support asset file bundling (concatenation
and minification) and multiple files should not be an issue.
Fix an accidental typo causing a syntax error
Set the default language to English
Remove trailing whitespace
Log an error and return if translations are missing
Merge pull request #12 from mitio/extract-translations

Nice work and new evolutions.