2018q2 release

This release covers everything from the 2018q1 release on 2018-04-05 and has been
tested with avr-gcc v7.3.0 and arm-none-eabi-gcc 2018q2 from Arm.

Breaking changes:

• No breaking changes.

Major features:

• Shared USART IRQ implementation for STM32F0/L0, fixes #88.
• Generic constexpr CAN bit timing calculations

Major fixes:

• Force UTF-8 encoding on Jinja templates, fixes #345.
• Fix missing ASCR register for STM32L4 GPIO.
• Correct formatting of float NaN and +-Inf values.
• Fix SoftwareSpiMaster::acquire() function.

Known bugs:

• GPIO connect on STM32F1 is still broken. See #178 for discussion.
  The API from MODM will not be backported to xpcc however.
• STM32F107 does not compile due to the HAL trying to remap USB. See #268.
• SCons build system emits multiple non-critical warnings. See #286.
• avr-gcc 7 complains about dynamic initialization put into program memory area. See #314.

New development board targets:

• Nucleo-L432KC board as nucleo_l432kc.
• Black Pill board as stm32f103c8t6_black_pill.

New device drivers:

• No new device drivers.

Many thanks to all our contributors:
A special shoutout to first timers (🎉🎊):

• Christopher Durand (@chris-durand)
• Lucas Mösch (@lmoesch) 🎉🎊
• Nick Sarten (@genbattle)
• Niklas Hauser (@salkinium)
• Raphael Lehmann (@rleh)
• Sascha Schade (@strongly-typed)

See our Changelog for more details.