Make Listen::Listener capable of listening to multiple directories

It also deprecates Listen::MultiListener since all the logic has been
generalized and moved to Listen::Listener.
Try to improve small_time_difference directoy_record helper
Better small_time_difference usage
Put all luck on our side
Travis test
Move rspec dev deps to gemspec
Try to fix small_time_difference spec helper
Re-add rspec in Gemfile
Less spec when HIGH_PRECISION_SUPPORTED is true
Compute the SHA1 sum only for regular files

Close #85