Update code base to ES6

[Jozo132]

**Includes ADS indexOffset fix caused 'read' 'write' and 'notify' to return values of 0 or false
Added '// @ts-check' for improved type checking
Replaced all 'var' types to 'const' and 'let' respectively
Replaced all 'function' types to 'const' arrow functions
Replaced all function inheritences of 'this' to pass as first parameter instead
Reduced code size by about 350 lines and structural imrpovements

Code examples must be updated becuase 'this' has to be replaced with your 'client' object
Example:

let client = ads.connect(amsConfig, () => {
    console.log(`Trying to read symbol:`, sym)

    //* // Read symbol
    client.read(sym, (err, handle) => {
        if (err) console.log(err)
        console.log('Response:', handle.value)
        client.end()
    })//*/

    /* // Add symbol to notify
    client.notify(sym)
    client.on('notification', handle => {
        console.log('Response:', handle.value);
    });//*/

})
client.on('error', e => {
    console.log(e)
})

[roccomuso]

Rebase needed.
Also, We might want to support old node versions.

[Jozo132]

Rebase needed.
Also, We might want to support old node versions.

First time I'm doing rebase .. I think I done it right. // Inexperienced GIT user here
I agree with the support for old node versions, so do as you see fit.
I'll be working on my fork and will do some more testing when I have spare time.
What I needed was ADS support for latest node versions on Windows systems, so that's how it got me here.

[roccomuso]

cool keep us updated