improved installer to avoid install of Gem libraries when doing incremental install (should significantly shorten incremental installs)

*cannot create two structs on the same canvas (otherwise how would you know which one you are talking to)?
*fixed regression where arbitrary font sizes were not honored by the drawsymbols and drawnumbers
*other minor clean-ups
*adjusted help file to reflect the new approach to font sizes

cleaned-up gpio external dependencies and supporting build script. made disis_gpio external into a comprehensive object for all things gpio, including pwm (both hardware and software) with sarcduino-like formatting and verbose output. The external requires sudo due to required access to /dev/mem and /sys/class/gpio

*added new externals for raspberry pi (disis_gpio for digital I/O and disis_spi for analog I/O using mcp3008 A/D converter). Provided preliminary help files. PWM on gpio needs to be tested further. So does the build script. TODO: add this to the installer, including 4755 chown_gpio script.

initial commit of the custom raspberry_pi externals supporting read/write from digital pins, analog pins (using shield with a mcp3008 A/D converter), and netsend~/netreceive~ optimized for rpi (net objects being copies of work done in the core pd)

removed clicks from multitap using variable delay (now has different temporary artifacts that actually sound pretty cool).

added one more k12 object and updated pd.tk to reflect new items in the k12 menu (including new minimum size)