parse AngraDB commands

As a user of the AngraDB client, it would be nice to have a parser to verify whether a specific command is valid or not, without the need to send a TCP request to the server. I think that the use of antlr would be a nice approach to deal with this issue. Please, take a look at: 

https://github.com/antlr/grammars-v4/blob/master/json/JSON.g4
