Implement CALLBACK_MSG_MODULE_IMPORTED.

Implement a callback to handle module data. It will be given a dictionary
which contains all the information stored in the module that is fit for
consumption. Things like functions are left out.

You can now do things like this:

```
import yara
import json
from pprint import pprint

def modules_callback(d):
    try:
        pprint(d)
        # This will not work if you have non-ASCII in your strings.
        #print json.dumps(d, indent=4)
    except Exception as e:
        print "Exception: %s" % e
    return yara.CALLBACK_CONTINUE

rules = yara.compile(source='import "pe" rule always_false { condition: false }')
with open('foo.exe') as f:
    rules.match(data=f.read(), modules_callback=modules_callback)
```

yara_callback was invoking Python's code without acquiring the global interpreter lock.

Starting with Python 3.2 hash functions should return Py_hash_t instead of long.