Move event threshold to a configurable option of the library

[elchininet]

The event threshold was a constant with 50, any consecutive events with a lower timestamp than this value were ignored. This pull request moves this value from a constant to a configurable option of the library, so it is possible to change it when one instantiates the class. With this pull request, the default value has been changed from 50 to 450, so if s lower value is needed it should be sent during instantiation.

config optional parameter

{
    retries?: number;
    delay?: number;
    eventThreshold?: number;
}

Parameter	Optional	Description
retries	yes	How many retries trying to find an element in the DOM tree before giving up (defaults to 100)
delay	yes	Delay between each retry trying to find an element in the DOM tree (defaults to 50)
eventThreshold	yes	Timestamp threshold to fire an event. If two consecutive events of the same type have a timestamp difference lower than this value, the second event will be ignored (defaults to 450)

[coveralls]

coverage: 100.0%. remained the same
when pulling 49eca5d on add_event_theshold_to_the_config
into 39c1151 on master.