Getting Bash-it to use the vendored composure

Fixed tests
Fixed install.sh and bash_it.sh
Added gitattributes to the vendor folder
Changed documentation

test/plugins/cmd-returned-notify.plugin.bats

@@ -2,7 +2,7 @@
 
 load ../test_helper
 load ../../lib/helpers
-load ../../lib/composure
+load "${BASH_IT}/vendor/github.com/erichs/composure/composure.sh"
 
 load ../../plugins/available/cmd-returned-notify.plugin