More support put in.

Now support for different formats of output.
So far I have tried 8N1, 8N2, 7E1, 7O1...

Fleshing out some of the missing pieces. - Need to test
copy NVIC execution priority from T3
Set priority per Serial object

RTS pin
CTS pin - only Serial3 supports
setRX, setTX - do nothing none of these have two pins defined.

Maybe temporary - but added from previous code ability to add second buffer for RX and TX

Used macros from OneWIre to fast set/clear the optional Transmitter Enable support.

Also fixed bug in ISR where was not properly looking for TC.  This will help make SerialX->flush() to work

This pass has some more support for using the Hardware Serial objects.

Including setting an ISR for doing both reads and writes,  sets up the FIFO,

Sitll lots more work to go.

Things like: fifo - need to better understand/set watermarks, also RX need to read in more than one byte if more available.

Also support fo r other features, like formats, 9 bit, ...

BUt at least you can do simple reads/writes

The T-LC and the new beta boards T3.6 and soon T3.5 hardware uarts have
ability to turn on 2 stop bits.  So enabled on these boards.

This is a WIP,  But I have now been able to create Serial6 on T3.4 beta,
and so far have tested Send/Receive basic stuff at 115200 and have tried
at several CPU speeds in MHZ (192, 216, 180, 120, 96)

https://forum.pjrc.com/threads/29446-Teensy-Hardware-Flow-Control-RTS-CTS?p=80425#post80425

https://groups.google.com/a/arduino.cc/d/msg/developers/ls3hkviFYM4/XWT2LbfzNcgJ

http://forum.pjrc.com/threads/26064-Large-transfers-over-serial-on-Teensy-3-1
http://forum.arduino.cc/index.php/topic,165574.0.html

http://forum.pjrc.com/threads/25494-Problems-with-HardwareSerial-Pointer-and-write