Multiple fixes and additions from real usage

Fixed URL handling: added path normalization; changed how `state`
distinguishes path from state name.
Fixed handling of the crossroads optional params (`:param:`).
Fixed Router.init without added states.
Renamed State.fullPath to getFullPath.
Added the params to state transition signals.
Added the params to initial state.
Added inner error for prereqs failures.
Added documentation comments.
Bumped version to 1.2.0 because of minor public API changes.