CMake: put everything into root module path if WITH_FIND_MODULE is enabled.

Until I made a fix for #2, KDevelop (and I hope also the other cool
IDEs) was able to parse the macros in `UseCorrade.cmake` and extract
targets and unit tests from calls to `corrade_add_test()`, among other
things. When I moved these macros from CMake's root module dir and put
them somewhere else, KDevelop wasn't able to find them anymore and thus
the targets didn't get detected, severely reducing the IDE experience
(i.e. no "one click" way to add new unit test target and run it through
debugger, no rebuild is triggered before running the executable etc.).

I can't expect KDevelop to reimplement `find_path()` command or
interpret all the CMake scripts to guess `CMAKE_MODULE_DIR` value from
which `UseCorrade.cmake` is included. Thus for now, until a better
solution is found, when `WITH_FIND_MODULE` is enabled, all the CMake
modules are installed into CMake's root module dir. This is completely
transparent to the user, no modiication in depending projects is needed
except for updating FindCorrade.cmake. But because this option is
disabled by default, the majority of existing code shouldn't be
affected.
Mention WITH_FIND_MODULE in building documentation.