Keepkey

Added unit test for 0x swap transactions.

Official release that includes updates since v6.0.3: Unit tests for added assets and vulnerability fixes, python 3 upgrade.

Bug Fixes
* Windows USB connectivity issues
* Don't fetch prevtx's on force-bip143 coins
* Re-enable DEBUGLINK commands through keepkectl

New Features
* Reduced dependency on the rlp package (only keepkeyctl depends on it now)
* Nano support (firmware not yet signed)
* Grostlecoin support (firmware not yet signed)
* CI, running against the emulator through CircleCI

Issues resolved:
* 18