Revamp image to build dependencies as part of stages

[PeterJohnson]

Fixes #17.

Stage 2 is fairly minimal, stage 3 builds/installs OpenCV and WPILib et al, and stage 4 builds/installs the FRCVision webdash and adds the vision examples.

Other changes:

• OpenCV compiled with ffmpeg, OpenBLAS, and libgtk (fixes Compile opencv with libgtk to allow interactive usage with imshow() #79, fixes Compile opencv allowing reading of video files #80)
• OpenBLAS added to image (fixes Suggestion: add ATLAS/OpenBLAS to image #65)
• C++ Makefile is more easily extensible (fixes C++ example Makefile doesn't handle additional deps gracefully #71)
• Sources for everything are bundled into image into /usr/src
• README updated (fixes Update README #16)
• pkg-config files for wpilibc et al are now installed and C++ Makefile uses them (if compiled local to Pi)
• Both dynamic and static libs are included in image

The only downside of all these changes (particularly the ffmpeg, OpenBLAS, and libgtk inclusion) is the image size is now over 3GB (800MB compressed). The previous image didn't quite fit on a 2GB card however.

[auscompgeek]

I'm slightly surprised you've gone to the effort of dealing with GitHub's tarballs for pynetworktables instead of using the tarball/wheel on PyPI.

[PeterJohnson]

It was really more for consistency than anything else. Sometimes we need to pull development versions rather than what's released.