rename writeBufferFree() to availableForWrite()
Add yield() for Teensy 2.0
Add extra NVIC_ISER registers
Use DMA_NUM_CHANNELS for index if allocation fails
Add volatile to all DMAChannel source address pointers
Use Serial2's FIFO on Teensy 3.1
Add transmitterEnable on Serial2 & Serial3
Update DMAChannel.h

Changed  DMABaseClass::source() and destination() to use p's address instead of its value in the if-clause. Taking the value could result in unwated read operations from I/O locations, resulting in data loss.