python: move module to separate directory

This allows submodules to be added under the 'hivex' namespace. A new
`pythonmoddir` variable was added for separating libhivexmod.so and the
Python hivex module.
python: export hive_types constants

Instead of using magic numbers or copying the numbers all over place,
make the hive type constants available so you can do:

    import hivex
    from hivex.hive_types import *

    h = hivex.Hivex ("../images/minimal", write = True)
    new_value = { "key": "abc", "t": REG_BINARY, "value": b"xyz" }
    h.node_set_value (h.root (), new_value)