change struct tag name

[ftKnox]

I have been using these changes in development and work fine. Ignore commit bbb73ad, I accidentally removed the json tag, but that still needs to be there as well. #44

[ewoutp]

Problem with this approach is that it does not work both ways (decoding & encoding).
In many place we use a standard json.Marshal/Unmarshal which does not take the arangodb tag into account.

[ewoutp]

If we add another tag (see problems above), it will be "use other tag if specified, otherwise use json tag" approach. With that it is not needed to duplicate all internally used tags.

[ftKnox]

Understood. So, I would put my vote in for supporting both a custom tag name and the standard json tag, since it would allow one to omit a struct value from the json encoding. (gorethink & mgo drivers work in this manner).