hydra_drive/package.xml: remove dependency on std_msgs
.gitignore: ignore root CMakeLists.txt (usu. symlinked)
moved node scripts into `nodes` subdirectories
hydra_arm: new package with basic arm control
hydra_drive/TwistSpeedMapper: removed stray semicolon
hydra_drive/TwistSpeedMapper: remove redundant pluginlib export declaration
new packages: hydra_imu (MinIMU9v3 drivers), hydra_common (utilities)

hydra_common currently includes only a version of the
`dynamic_reconfigure::Server<...>` template definition modified to play
nicely with nodelets.

hydra_imu contains both Arduino driver code and a small set of nodelets
to enable retrieving and processing data from the MinIMU9v3 units.
One of the salient features of this implementation is dynamic scale
selection in the Arduino code, which allows higher-resolution sensor
data when the measured quantity is small and higher-range when it's not.
renamed: hydra_drive/arduino/{hydra_drive=>HydraDriveControl}
hydra_drive: new Arduino sketch HydraDriveEncoder

Not _quite_ complete, and not integrated into the HydraDriveControl sketch.
hydra_drive: circuit for the drive-encoder driver boards