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)