Added documentation for optional Serial2 USART on Spark pins D1(TX)/D0(RX).
Minor edits
Added SYNTAX section for Serial2
Doc tweak to install notes
added how to actually control the onboard LED

under "Blink and LED with Tinker"
Merge pull request #160 from gruvin/feature/serial2

Documentation for feature/serial2 -- Issue 89
Merged, fixed conflict