introduce timestamp_t as uint64_t to manage Ticker; reimplement nRF51 us_ticker using RTC

These changes allow us to re-implement us-Ticker using the RTC for the nRF51822. There are two major themes here:

* timestamp_t as an abstraction for time values managed by Ticker. Using uint64_t for timestamp_t allows a wraparound-free Ticker. This change forces us to update the definitions of usTicker for all platforms; but the changes beyond nRF51822 aren't major.
* reduce power consumption on the nRF51822 by removing the need for the high-frequency processor timer; and reimplementing it using the RTC.

I've also replaced high-frequency clock with low-frequency external clock during system startup of the nRF51822. This brings a major win in power consumption.
us_ticker uses the app_timer module to setup a single-shot timer callback based on RTC. The Ticker class retriggers this single-shot timer after every callback. Unlike a hardware timer, interrupts from the app_timer's timer don't need to be disabled.

But I shouldn't assume that clear_interrupt() won't be called on an active timer. This can now happen if the Ticker goes out of scope and its destrutor is called. So I'll add an implementation for clear_interrupt().

Thanks Martin!