Rename getContactByEmail to getContactsByEmail

The getContactByEmail function actually returns an array of contacts if
more than one is matched because it is using "get_contacts" from the
GetResponse API instead of "get_contact". Therefore, the appropriate
name for the function should be getContactsByEmail() even though the
return object may only contain a single record
Expose $operator in getContactsByEmail

The $operator variable is now exposed in the getContactsByEmail()
function and defaults to 'CONTAINS'. It can be overridden by 'EQUALS' or
any other valid text operator from the GetResponse API.