Added support to extract non-delimited string from attributes

Regarding this issue on SO http://stackoverflow.com/questions/19792782/angularjs-gettext-how-to-translate-text-in-attributes, <code>nggettext_extract</code> does not extract strings that are not delimited.

This pull request added the support to extract the strings that are not delimited. This is needed since plenty of directives (especially custom directives) take an expression without delimiters.

However the remaining issue is that strings that are not expression will be extracted as well.