Describe CMake integration
Remove unneeded CMake command

Definitions are already included with add_backward.
Fix message type

Whoops, there is no INFO message type. Make it a WARNING instead.
Make feature detection values internal

Previously, the "feature detection options" (STACK_DETAILS_* and STACK_WALKING_*) had to be manually set by an user, but now, we (attempt) to detect libraries based on find_library, thus we don't need those to be cache values anymore. Libraries in non-default paths can be manually added in the corresponding _PATH cache value and for testing purposes, users can also disable libraries by supplying an empty string.
Fix CMake style

I erred concerning CMake variable naming conventions.
Mark most cache values as advanced

To avoid confusion, hide some configuration value from the normal user.
Revert 0660344

Turns out I was wrong, we do need that command.
Merge pull request #20 from hesiod/master

Documentation