should not be kept indefinitely, since sources may've been changed already
ci cache dir is expected to only survive for the current build session,
prefer similar behaviour for local build tests
- just pass `-include ${cfg}` to the compiler instead of a
temporary custom.h. also do `-DDEVICE=...` and `-DMANUFACTURER=...`
- rework test_build to print using `logging` module
- report leftover configurations, after the failed one exits
- prettify the output and highlight relevant info instead of the whole lines
- remove .travis.yml
- ./travis_{script,install}.sh -> ./ci_{script,install}.sh
- flush stdout in test script to see logs more promptly
- adjust git config to ignore 'detachedHead' warnings
- general conversion from .ino modules into a separate .cpp files
- clean-up internal headers, place libraries into .h. guard .cpp with _SUPPORT flags
- fix some instances of shared variables instead of public methods
- tweak build system to still build a single source file via os environment variable ESPURNA_BUILD_SINGLE_SOURCE
