Allowing underscores in numbers and long numbers.

Hello,

I've managed to create a patch that implements issue #11. Which makes this possible:
```
1_234_567  # => 1234567
1_234_L    # => 1234L
```

Full disclosure: I am not a Java expert and digged into Golo to (re-)learn it, so if you start reviewing this pull-request and spot rookie mistakes, I'll blame it on this. :smiley_cat:

Thanks in advance for your feedbacks!