1.1.5 - RTC millis option, timer-related fixes

This release introduces a new option to use the RTC timer as the clock source for millis(). When this is used, micros() is disabled. If standby sleep is used (which uses only slightly more power than powerdown), millis will keep count during sleep (but a small extra step is needed to ensure that it stays asleep when the millis interrupt wakes it, see the README). This update also makes some small improvements in flash usage within the timing code, and fixes the option to disable millis/micros entirely to save flash (this was broken in 1.1.3). Finally, the variant.c file was eliminated and its contents moved elsewhere to improve maintainability. Part-specific documentation pages have also been updated.

This is the version for board manager installation.