Remove the use of $terminal global variable

[abinoam]

To maintain minimum back compatibility we will follow these steps at the internals:

• Create a HighLine instance after definition of HighLine class
• Saves it at HighLine.default_instance
• Everything we have been doing with global $terminal now we do with HighLine.default_instance
• Take the opportunity to group related methods together to easier code reading

Although we're still relying on a global state at least now we're namespaced on HighLine and we now avoid name collisions that have happened at the past with $terminal global var (as terminal is a very common word).

[abinoam]

@JEG2 as I said at the top

Although we're still relying on a global state at least now we're namespaced on HighLine and we now avoid name collisions that have happened at the past with $terminal global var (as terminal is a very common word).

Basically it deprecates the use of $terminal and uses HighLine.default_instance.

So... merging.

[JEG2]

Love this.