Replaced all KeyErrors by actual exceptions, fixes #247
Removed load and join community, handled by the community constructor
now.
Update my_member in community table when a different one is passed to
the constructor.
Create identity at end of constructor
Load database version and id
No more member by database id cache, fixed tracker
When inserting a member with a private key, also derive the public key
Require a public_key whenever a private key is going to be inserted
Merge pull request #252 from NielsZeilemaker/remove_keyerror

Replaced all KeyErrors by actual exceptions, fixes #247