add support for nRF51-DK
Renamed TARGET_GHI_MBUINO to TARGET_OC_MBUINO to fix an issue with the online compiler.
Merge pull request #469 from Experiment-626/master

Renamed TARGET_GHI_MBUINO to TARGET_OC_MBUINO to fix an issue with the o...
add app_common header files for Nordic targets
[NUCLEOs] enhance i2c api
add some more files related to app_common/ to the Nordic Lib
Replace the 16mhz clock with a low-freqncy external crystal based clock source at statup.
This results in a big win in power consumption.
add a missing call to us_ticker_disable_interrupt from us_ticker_remove_event.
reimplement the micro-second ticker on nRF51822 using the app_timer module (RTC)
add a minor comment to explain some complex branching in us_ticker_irq_handler()
add a cachedInterruptTimestamp to us_ticker_set_interrupt() to short-circuit duplicate calls.
introduce a timestamp_t to track timeouts for TimerEvents.
The previous type used for this purpose was unsigned int, which was both
un-portable and liable to wrap-around.
updated call to APP_TIMER_INIT() to use contants from projectconfig.h
simplify the algorithm which keeps only one app_timer timer running.
white space diffs.
update definitions for us_ticker_set_interrupt() across all platforms to use timestamp_t
use constants from projectconfig.h as arguments for APP_TIMER_INIT()
minor simplification of a reused expression
use timestamp_t for Ticker::_delay
add a destructor for Ticker.