Fast SPI pin changes
- added defines to map ports and bits for RESET, DC and SS.
- added macros to quickly control RESET, DC and SS using
  the internal weak pullup for high.
Eliminate compiler warnings
- Fixed use of = instead of == in "if" statment.
- Fixed uninitialised count variable in "for" loop.
- Removed unused variables.
- Replaced line splice characters (backslash) at end of comments.
Help further reduce overdriving display inputs
(and fixed a few spelling errors in comments)
- Use fast pin change macros for RESET, SS and DC.
- Implement MVSPI.packetBegin() and MVSPI.packetEnd() to allow sending
  multiple byte "packets" while SS remains enabled.
- Add 2 and 3 byte command() functions to send multiple command bytes in
  one packet.
- Modify clear() and display() functions to send commands and data in
  packets.
- Do the "wait for SPI transfer complete" just before sending the next
  byte (plus after the last or only byte) in a packet, so code can be
  executed while bytes are being sent.
- Disable SPI mode when not transmitting, so MOSI (and SCK) will go low.
- Keep DC low except when sending data bytes.
- Change MVSPI.transfer() to void since nothing can be received from
  the display.
Added uView.end() to power off the display.
In uView.begin() don't send commands that default to desired values.
Improve speed of display() and clear() functions
by using horizontal addressing mode.
Merge branch 'master' of https://github.com/geekammo/MicroView-Arduino-Library