Contracts extension + bug fixes

This release introduces the EthOn Contracts extension.
Many tracked issues have been fixed. For deteails, please see the issue tracker. Additionally, some term descriptions have been improved.

Some issue fixes

fixed issues #17, #20, #21, #24, #25, #28, #29, #30, #31, #33, #34, #35

Mostly added properties to concepts that were forgotten.
One other major change is that ethon:Uncle is not a subclass of ethon:Block anymore.
For details, please look at the commits. As this is a pre-release of EthOn and things are changing quickly, no full changelog is provided. From version 1.0.0 on, this will change.

Initial public release

This is the first public release of EthOn.

(See issue #3 for the versioning scheme used.)