arbitrary backend <-> crossfilter bridge

This library implements enough of the crossfilter API and an asynchronous commit hook so that (in theory) any query engine can be connected to dc.js

Currently the query engine supported is Nanocubes (this library was originally called "nanofilter"): use xf.engine(xfilter.nanocube_queries) to connect with a Nanocubes server (demo).

Another example engine xfilter.fgb_queries() assumes the server takes queries that look like

{
    "filter": {
        "column": [
            "val1",
            "val2"
        ]
    },
    "groupby": [
        "column2"
    ]
}

The intent is that this library can be generalized to support any query engine.