adding code relevant to cc2538
adding my code so far for cc2538em
Added install to cc2538em platform
also added to rules
fixed pin read issue
implemented TimerMilli

Need to add microsecond timers and the rest of the 32khz and milli
family.
implemented more of the timer features
More work on cc2538 timers

Starting add Micro precision. Stopped due to my inablility to program
the cc2538em any longer.
cc2538 microsecond timers work
Rounded out cc2538 timer code

Added more microsecond components and added a readme
Cleaned up cc2538 timer nits
Merge branch 'cc2538'