Adding i18n support - Fixes #2

I've been needing this functionality and saw that there was already a ticket open for it here, so this is implementing it.

To make this work I added the locales and messageFilePathFormat options for the corresponding options of SoyToJsSrcCompiler.jar and started using them. That's the first commit.

The second commit is guaranteeing that compiling templates to multiple locales will work well with soynode. It's a more complicated change, as it involves having multiple vms, since we don't want different versions of the same template to run on the same vm, overriding each other. It also changes the concatenation logic to output a file for each language, when there are more than one. So now users can compile templates on multiple languages, and then they just need to specify which language they want to render them on when using them.

Let me know what you think of this, I believe it will be very useful :)
you can use {string=} to indicate that the argument is optional