Add tests for increased coverage

[ryanio]

This PR adds tests for increased code coverage:

• Adds BlockHeader validation test cases:
  • should validate a genesis block header
  • should validate a valid block header
  • should not validate a block header with invalid difficulty
  • should not validate a block header with invalid gas limit
  • should not validate a block header with invalid height
  • should not validate a block header with invalid timestamp
  • should not validate a block header with invalid parent
  • should not validate a header with invalid amount of extra data
• Adds test for blockHeaderFromRpc:
  • should create a block header with the correct hash
• Moves testdata json files to their own subdir to unclutter the parent test dir
• Updates karma to run in both headless firefox and chrome
• Modernizes the test and coverage command
• Normalizes the readme badge style