v0.4.0

Version 0.4.0

Changelog:

- NMEA
   - added class 'waypoint'.
   - added query functions for manufacturer information.
   - type for padding bits of the payload changed to uint32_t.
   - cleanup of public API of sentences.
   - constructing sentence objects differently, gains about 10% performance.
   - minor refactoring of checksum rendering, also faster.
   - preformance improvements in string splitting.
   - added benchmarks for some NMEA sentences, testing nmea::make_sentence

 - AIS:
   - type for padding bits of the payload changed to uint32_t.
   - added benchmarking of AIS message decoding.

 - Doc:
   - added GCC-6 and Clang-3.8 as supported/tested compilers.
   - exclude the namespace 'detail' from generated documentation.

 - Build:
   - IO is now optional, use flag 'ENABLE_IO'.
   - minor cleanup for warning hell.
   - minor warnings cleanup

 - Dev:
   - added Clang-3.8 debug build
   - added GCC-6 debug build
   - disabled travis-ci clang builds, llvm.org/apt is out of order at the moment.