json output

Add output of explicit strategies to the `json` format described [here](http://slivingston.github.io/gr1c/md_formats.html), because writing a custom parser (or one based on a parser generator) for the `jtlv` output format everywhere that a strategy needs to be loaded can be avoided. Otherwise, importing an existing parser is tempting, leading to unnecessary dependencies.

If in `python` the `stdout` of `slugs` is stored in variable `out`, then constructing a `networkx` graph from the `json` output is simple and succinct:

```python
import json
import networkx as nx

dout = json.loads(out)
 g = nx.DiGraph()
    dvars = dout['variables']
    for stru, d in dout['nodes'].iteritems():
        u = int(stru)
        state = {k: v for k, v in zip(dvars, d['state'])}
        g.add_node(u, state=state)
        for v in d['trans']:
            g.add_edge(u, v)
```

The only thing that is missing is a command line option to switch between `json` and `jtlv`, for backwards compatibility. Preprocessor conditionals have been used for now in `extensions/ExtractExplicitStrategy.hpp`, because I am not sure what is the best way to add the argument to the existing mechanism of parsing arguments.

Also, the extensions have been relocated in their own directory, to emphasize what the core sources are, and make it easier to navigate them.