add change event

Test file: tests/tests.js
# There should be two working prettySelect boxes.
Initial captured successfully.
PASS Select boxes are hidden
PASS There are two prettySelect containers
PASS The drop elements are hidden
PASS Wrapper class setting was applied correctly
PASS The drop elements are visible after a click
Post-click screen captured successfully.
PASS Found "Value 3" within the selector "#select-1-ps .ps-label"
PASS "select" input field has the value "value3"
PASS "Selected" attribute for selector #1 has been moved off the "value2" option
PASS "Selected" attribute for selector #1 has been moved to "value3" option
PASS Found "Value 6" within the selector "#select-2-ps .ps-label"
PASS "other" input field has the value "value6"
PASS "Selected" attribute for selector #2 has been moved to "value6" option
Final screen captured successfully.

Done, without errors.