util/Signal: some doc-comment additions/improvements
util/Signal: renamed `add` => `connect`
util/SignalAction: renamed `cancel` => `disconnect`
input/Axis: added missing #include <algorithm>
Some minor improvements to util/Signal

re: handler invocation, sub-type typedefs, comments, etc.
src/CMakeLists.txt: crisp-input requires boost_system library
input/MappedEventSource: switched to the new Signal implementation