minor changes

- added Block.duplicateAsHeader() to support storing blocks with only header information (preparation for dartcoin.wire)
- added VarStr
- added some @override tags
- added magic value to network parameters to prepare for dartcoin.wire
Implemented the basic messages of the Bitcoin protocol in dartcoin.wire subpackage.

With basic I mean that I skipped the Bloom filter-related ones that are added with BIP0037.