SPI Driver API bugfixes and cleanup, part 2 (ARMmbed#180)

* Make it so that SPI::select() works correctly with async stuff

* Add more overloads for SPI functions, make sure that SPI::abort_transfer() correctly toggles CS

* SPI: Implement reference counting so that DMA channels get freed properly

* Fix initialization of SPI peripheral structures

* Update docs a bit

* Use a mutex to protect SPI::_peripherals instead of a critical section, because spi_free() may not be ISR safe

* Style fixes

Author: Jamie Smith