R bindings for ev3dev - preliminary version

Compatibility with ev3dev-jessie-2014-10-07 (pre-release).

Implementation follows wrapper-specification.md and naming conventions of cpp bindings and R language
(e.g. Position for getter and SetPosition for setter, Run, Stop, Running for motor, ...).

What's working:
- motors
- sensors
- leds
- power supply

What was tested:
- motors, mainly in relative and absolute positioning mode
- sensors (infrared)
- leds (setting, getting brightness)
- power supply (getting all the properties)

TO DO:
- connected property only set correctly during ininital creating of device (not set to false after errors in later use)
- possible performance issues for FloatValue (reading dp every time)
- function Position is creating a new generic function for ‘Position’ in the global environment (instead builtin R Position)
- use explicit integers where possible (e.g. 1L instead of 1), no FPU!
- stick to S3/S4/R5 classes only (one of), S4 now so copy constructible initializers would be better
- documentation
- some more constants should be defined (for devices, sensors, modes, etc)