Various enhancements for Babel

Hi Martin, 
I am hereby proposing some minor enhancements for Babel.
First of all, I'd like to say I like its simplicity. Therefore, I tried to stick to that spirit in the following changes committed.

Most of the changes are syntax enhancements, error checking on loading or swicthing to locale files. Of course, users can define their locale file names without any restriction, in its current state. But i suggest (for the given examples in the Readme), to stick to the most common names one can find for locale files (that is `en-US` instead of `eng-US`; or `fr-FR` instead of `fre-FR`). But that is just my opinion.

I have also made `babel.mergeTable` an internal function, as it just looks like an helper added for mixin purposes. So it shouldn't be exposed as a public function, in my humble opinion.

Readme was also updated, with some fixes in some snippets and minor typos fixes.
Best regards,
Roland.