Improve compatibility and compliance

1. Support for infinitie and NaN printing for better compliance
with standard library functions.
2. Add support for library installation to CMake configuration.
3. Clean up CMake configuration code.
4. Fix target confusions when including as a CMake subproject.
5. Adjust pre- and postconditions of all the functions.
6. Remove excessive checks and linearize some code paths.
7. Fix documentation in several places.

Fix bug in lower boundary computations

Fix lower boundary computation to make
conversions of powers of two correct.
Many thanks to Alexander Bolz (@abolz) for help
with this problem.

Make Floaxie pure header-only library in terms of CMake

1. Add `INTERFACE_LIBRARY` target.
2. Make all the remaining building targets optional.
3. Sort out the compiler settings.
4. Add description on how to easily add Floaxie to any CMake project.

Minor fixes and better test coverage

Fixes:
* little branching optimisation in Krosh code
* strong requirement for C  14 in root `CMakeLists.txt`
* fully qualify `<cmath>` functions from std, to avoid collisions with built-in
Glibc ones
* fine tuned warning switches for MSVC compiler

Tests added to improve code coverage:
* increment and decrement in `diy_fp`
* default string parsing callbacks and error state processing
* under- and overflows processing in `diy_fp::downsample()`
* several tests added for `atof()` alternative code paths

Builds using Travis CI (for GCC and Clang) and AppVeyor (for MSVC) and code
coverage estimation using Codecov were set up.

32-bit `float` support and better `strtod` compatibility

Significant update to version 1, containing the following:
- 32-bit `float` type support
- overflow/underflow, NaN and infinity proper processing to be more compatible
with `strtod`-like C standard library functions
- fixed building in MSVC

Bugfix update to version 1.0.0

Changes:
- fix error happened when parsing
representation of zero value ("0.0" and likes)
- add wide character support

First release

Generally all the planned functionality and algorithms
are there, front-end API is more or less C standard
library compliant and not expected to be changed. So,
here's the first release.