Extract line numbers from JavaScript

Esprima also supports column numbers, which would be pretty trivial to add to this code, but I've never seen that done in PO files. Let me know if that's something you'd want.

If you consider the generated POT file to be part of angular-gettext-tools's API, then this change requires a major version bump. Or you can turn off line numbers by default, but I think they're useful and should be on.
I consider locking down the fixtures a good thing. Ideally we never touch them and only add new ones.

The whole reason for test fixtures is avoiding regressions: what better way to ensure backwards compatibility than by strictly staying in the past :-)