When rules are passed as strings, the line number would not be shown
when input as a string. The information is available however.

If it is not a file like object, the function would return -1. This
would eventually lead to a segmention fault later on.

The "scan" method was returning a error when the "externals" dictionary has keys that weren't previously passed to "compile". Ignore those extra keys to avoid regression errors.

Show appropriate error message when an external variable is redefined with a different type. See issue #12.

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.