Refactor build configuration into `site_scons` module

Refactor the creation of an Arduino build-context into the `site_scons.arduino_build` module.  Among other things, this makes it possible to build several variants of projects, by specifying a different build directory for each variant.  It should also make it easier to incorporate Arduino builds into other projects.

I also plan to use this as a starting point to add support for the [Due][1].

[1]: http://arduino.cc/en/Guide/ArduinoDue