Improved message fixtures and data driven testing

Chained on #186.

This pull request improves the testing coverage of `Message` slightly, but mostly moves all the testing data to JSON and encourages the use of data driven testing.
The test data is almost entirely extracted from the original data except for `signing.supports alternate networks`, the signature was changed to the uncompressed form.

Comparison link: https://github.com/dcousens/bitcoinjs-lib/compare/jsontests...messagefixtures