Rectoring of RTC::Timer (first increment).
Back to absolute time in RTC::Timer. Some further cleanup.
Remove RTC::Timer latency, add example

Example includes relative and absolute Timer usage (expire_after and
expire_at, respectively).
Merge branch 'master' of https://github.com/SlashDevin/Cosa into SlashDevin-master

Conflicts:
	cores/cosa/Cosa/RTC.cpp
	cores/cosa/Cosa/RTC.hh
Fixing problem with CosaBenchmarkRTC after latest change to RTC::Timer.