Implement new ArangoDB functionality support: Async requests

[frankmayer]

For driver version 1.4 we need to implement async request functionality described here:

• http://www.arangodb.org/manuals/1.4/HttpJob.html
• https://github.com/triAGENS/api-implementors/issues/19

[frankmayer]

Small update.
It turns out, this one needs a lot more work than expected.

With the current structure of the client there need to be a lot of changes in several classes, to make it work.

So, there are two approaches to it.

1. leave this functionality for the "Core" version which is kind of very slowly developing at the moment. No ETA on that, sorry.

2. Implement this functionality here. This will need changes in almost all classes, unless I am missing something.

[lpj145]

bump! this feature have any update ?