Bitstamp Standalone API - new submodule

Hi,

This version now includes all the necessary code (the external dependency is gone). It can be built in one step (mvn clean install).

I didn't want to push directly into develop since I wanted to check first if the way I did it seems fine. I added a new submodule, xchange-bitstamp-standalone, that is the "pure" implementation of the Bitstamp API with no dependencies on XChange. (The xchange-bitstamp module then depends on xchange-bitstamp-standalone.  xchange-bitstamp's role is to function as an adapter from  xchange-bitstamp-standalone to the unified xchange API.)

The reasons I did this (as opposed to just including all the code in a single xchange-bitstamp module) are these:
- Some people might need just to connect to Bitstamp (and no other exchanges) and don't need a dependency on the rest of xchange at all (as a matter of fact, I'm one of these people, in the project where this bitstamp-api originated).
- This allows new functionality (like bitstamp codes, withdrawals, ...) to be added to xchange-bitstamp-standalone that can be used immediately without adapting it to the xchange API (in xchange-bitstamp).
- I generally prefer more modularity & code decupling to less.

Let me know what you think.