Magnum.h now doesn't include anything except OpenGL headers, thus
changes in Math library don't trigger recompilation of everything, but
only of things really depending on it.
Math constants moved to separate file for similar reasons, de-inlined
some functions to remove the need for some #includes.
* All pages are now named with lowercased letters with dashes between
letters.
* Added coding style guidelines.
* Extended building documentation with guide how to download Magnum.
Reusing macros MAGNUM_EXPORT and friends already used for Windows
builds. For exported classes added MAGNUM_LOCAL to private members which
are not referenced from any inline function. Added explicit
MAGNUM_EXPORT to private members which are referenced. CMake provides
its own macro <target>_EXPORTS, using that instead of
set_target_properties().