* pmsleep refactor
* Shortened swtmr disabled message
* Added swtimer debug module option to user_modules.h.
* Added comments to user_config.h.
* Fixed error in documentation for node.sleep()
* remove blank sntp.c that got added in during rebase onto dev(6218b92)
* Added #ifdefs around SWTIMER_REG_CB to prevent inclusion of disabled
code
* Change upper limit for timer in `tmr_register` and `tmr_interval` to reflect new limit in SDK 1.5.3.
* Change documentation for `tmr.alarm()`, `tmr.interval()` and `tmr.register()` to match source.
* Improve error reporting to be more descriptive.