"Export" key components of the library

[dbluhm]

The didcomm-python library should "export" the key components of the library from it's main package __init__.py.

This will make it easier to consume and direct people to the types and objects they're most likely interested in. For example, rather than:

from didcomm.pack_encrypted import pack_encrypted
from didcomm.unpack import unpack
from didcomm.message import Message, Attachment
# ... etc

We should make these importable as:

from didcomm import pack_encrypted, unpack, Message, Attachment # etc. etc.

This will create a more cleanly defined "public interface" for the package and help people discover key entry points to the library.