split compression into main method and helper
introduce a compress_ptr method
change signature of helper to be explicit about pointer

To ensure that anyone reading the code, realizes that the first argument of the
helper should be an address, we change the type from 'unsigned int' to 'void
*'. This also means the cast from 'unsigned long' (or whatever type it ends up
being) to 'void *' happens only in the 'compress_ptr' function.
use PyLong_AsVoidPtr as suggested by pv
check the return value of PyLong_AsVoidPtr for NULL
use size_t during compression
add a toplevel compress_ptr function
change length -> items
fix documentation
Merge pull request #16 from esc/feature/compress_from_address

Feature/compress from address