fix for visible/waitUntilVisible when selector matches multiple elements
more functional implementation of visible method
add new parameter settings to sendAJAX, so we can override the default content type when sending out ajax requests

Some sites don't accept the default content-type "application/x-www-form-urlencoded", must send out the specific content-type.
so we need the extra settings to override the default content-type setting in current sendAJAX implementation.
Merge pull request #432 from rigoleto/fix-for-visible-with-multiple-elements

fix for visible/waitUntilVisible when selector matches multiple elements
updated CHANGELOG
Merge pull request #440 from RockLi/improve-send-ajax-function

add new parameter settings to sendAJAX function
fixes #457 - fill should not emit console messages