Named churn msgs, don't allow 0 time for ON/OFF
ChurnNode: Correct call of superclass in startApp()
OfflineDetectionNode: Various fixes for pending map

* Added start/stop/crashApp() functions
* Correct initialization of pending map
* Named messages
* Use std::make_pair() (for shorter code)
OfflineDetectionNode: Bugfix (wrong variable name), destructor
ChurnNode: Signals
Distributions: Lognormal, Weibull (+ minor optimizations)
.gitignore: Executable in top directory