Add antlr language .g4

[Dhiraj240]

The keywords for both the versions are same and there is just a change of extensions where the newer version automatically constructs the parse tree.
So i will be sending PR on this, if its okay.

[Dhiraj240]

Please assign it to me.

[jayvdb]

Do .g4 first , and then .g later.

I am very very interested in knowing if there are any significant grammar differences between the two languages.

[Dhiraj240]

Okay, i will edit this issue and open other issue with .g after completion of this one.