Update compiler and dependency info in README.
Add link to Google Groups to documentation.
Finally do some documentation.
modules: updated FindMagnum.cmake and FindCorrade.cmake.
Making includes absolute, part 1: moving files around.

Everything what was in src/ is now in src/Magnum. Only moving the files,
no other change was made.
Making includes absolute, part 2: make it compile again.

Added new root CMakeLists.txt file. Currently it compiles only if
MAGNUM_BUILD_DEPRECATED is enabled.
Making includes absolute, part 3: updated FindMagnumIntegration.cmake.
modules: updated FindCorrade.cmake and FindMagnum.cmake.
Making includes absolute, part 4: absolute includes everywhere.

The only place where they aren't absolute is when header is included
from corresponding source file.
Making includes absolute, part 5: better filenames for internal files.

As we are now using absolute includes, there is no need to prefix
everything with "magnum<Namespace>".
Making includes absolute, part 6: fix documentation.
doc: documenting directories.

Noting which dir is part of which library.
modules: updated CMake modules.
Happy new year!
Removed nonsense from CONTRIBUTING.md.

We don't have any Doxyfile, the only one is part of Magnum.
modules: updated FindCorrade.cmake from Corrade repository.
modules: once more.