Added weekday defines

Added weekday defines to simplify usage and avoid errors. See URTCLIB_WEEKDAY_XXXXXX on src/uRTCLib.h or https://naguissa.github.io/uRTCLib_doc_and_extras/uRTCLib_8h.html#a9fa4689ef453aa71f8b0f30c506eab73

Add deepSleep using RTC example

Add DeepSleep using RTC to wakeup example in response to #20

Power lost support for DS1307

Power lost support for DS1307, thanks to @QoBo68

Fix century bit on year read

Fix century bit interfering with year read - Thanks @davidhbrown for #17 and #18

Hardcoded century to 1 and added little note on RTC->set->year parameter

Hardcoded century to 1 in RTC->set - thanks @meltingSnowdrift for #16

Fix example on ST's Cube STM32 and split doc and extras

• Fixed examples on ST's cube STM32. Thanks @prosper00 for #15
• Moved doc and extras to other repository, https://github.com/Naguissa/uRTCLib_doc_and_extras

Fix alarms bugs

Fix alarms bugs thanks to @grepis

Added serial RTC set example by @ArminJo

Added serial RTC set example by @ArminJo
Fixed a compiler warning by @ArminJo
Integrated Travis-CI
Added Travis-CI status badge at README file

Doxygen documentation

Update source code documentatuion and generate Doxygen documentation; available on doc/index.html

Fixed negative temperatures

Fixed negative temperature calculation. Thanks @bxparks