encode_cstring fix for longs and int

Hey Martin,

As I was using your bson parser in a project, I noticed it didn't handle
long type keys to well. I was given a "can't add str and long objects" error.
My data structure being:

{320L: {'title': 'testing, 'lat': 20.0, 'lng': 50.0, 'id': 320L}, 
 300L: {'title': 'testing', 'lat': 20.0, 'lng': 50.0, 'id': 300L}}


This led me to look into encode_cstring
and make the following check for long or int types.

Line 108 in encode_cstring/1:

if isinstance(value, long) or isinstance(value, int):
	value = str(value)

I have attached it in my forked version of bsonpy which I have made a pull
request on over github.

I hope I could help in anyway.

Thanks,
Nadir