Add 'Dev' build type. Remove need for OsmiumOptions.cmake.
Fix for Windows compatibility.

Fixes #59.
Switch include used in Reader test.

It only uses XML OSM files not PBF.
CMake config: Only link tests with the libraries they need.

This commit also gives unit tests and data tests separate config variables
BUILD_UNIT_TESTS and BUILD_DATA_TESTS so you can only compile all unit tests
and/or all data tests, respectively.
CMake config: Add threading support for all IO functions, not only PBF.
Fix header file checking.

The point of the header file checking is to compile each header on its own.
Thats what the code in the Makefile did, but the CMake equivalent did not.
This is now fixed.
CMake config: Do not build header checks by default.
Bugfix: Wrong string return.

We were returning a string, moving it, which leaves m_str in an undefined state.
But we potentially use it later again.
Remove superfluous std::moves on return statements.

Remove these because the disable Return Value Optimization.
CMake config: Fix test dependency detection

Also all skipped tests are added to OSMIUM_SKIPPED_TESTS cache variable.
CMake config: Formatting.