Martypy

- Automatic Marty discovery when connecting over USB

- Improved default parameters for `wiggle()` and `sidestep()`

- **Blocking Mode**
- You can now select whether movement commands are blocking or non-blocking.
- The default behaviour changed from non-blocking to blocking.
- New `stand_straight()` method matching the "Stand straight in <...>" Scratch block.
- Marty V1 can `dance()` now!

- Better defaults for movement commands
- Various bugfixes including:
- The `walk()` command alternates steps by default
- You can use one `sidestep()` command to take multiple side-steps
- The `twist` argument to the `kick()` command now works
- We got a bit confused about which is left and which is right but Marty explained it to us!

: Fixes a bug when connecting to Marty V1

: Added Marty V2 support
: Removed unnecessary imports
: Dropped Python 2 support