Fix markdown formatting for diagram of the bit structure.
Fix so the generated txids are lexically sortable over time.

The first few bytes of the txid didn't contain the data for the entire
timestamp, so it was causing the generated txid to not be lexically
sortable as intended.

Add tests to try and better ensure that the txids are in fact sortable
and increment over time as expected.
Attempt to fix travis-ci tests by upgrading Valgrind on travis.
Merge pull request #4 from GUI/travis-valgrind-update

Fix so tests pass on travis-ci
Merge pull request #3 from GUI/sortable-fix

Fix so the generated txids are lexically sortable over time
Merge pull request #2 from GUI/readme-formatting

Fix markdown formatting for diagram of the bit structure