Added event handler to process the stream end signal

[jmhomedes-ob]

Added call to "done()" function in order to close ended stream and propagate error signal, if it applies, to CI systems like Jenkins.

[jmhomedes-ob]

callback() is called inside a timeout because I detected that when it's called with an error it interrupts de generation of HTML gemini report. It's working this way, but I'm sure the code is improveable and reviews/suggestions accepted.

Now you can update documentation by naming that with this change users will be able to do:

gulp
.src('src', { read: false })
.pipe(visTest.test({
    ...
}))
.on('end', function(){
    // will execute allways
})
.on('error', function(error){
    // will execute only when error signal (some test has failed)
});