Implement RubySpark::Variable

[kalkov]

It will be nice to have a Variable class, so we can access other keys, not only the result one. Like

data = RubySpark::Variable.new('core_id')

or

core = RubySpark::Core.new('core_id')
data = RubySpark::Variable.new(core)

So we can use :

data.connected
data.last_heard

[efatsi]

could those just be methods on the Core class?

That's the approach I took with the name and variable function lists. I'll flesh this out and add additional instance variable that get updated with each API request.

[nixpulvis]

Having the ability to abstract past the single Core for events makes sense, variables exposed through a single core should stay inside the Core class IMO.

This commit: https://github.com/lhalff/ruby_spark/commit/1191fe8df27ac36597d2d29780dfa4528cb3ba00 proposes a way to store and keep core info up to date.