GSOC2018][RubyPlot-GR][20-6-2018]xReworked the function classes and added display function

[Pranav Garg]

Greetings Everyone,

As per the discussion yesterday I have reworked the function classes which are to be pushed to the task list into a GR Level. I have also added a display function so that user can view the graph, in addition, to directly saving that.

Also Rubocop is awesome! I will make using a code linter a part of my process.

Here is the link to the commit :

https://github.com/pgtgrly/GRruby-extension/commit/ae6137c2cf0b5b93fdec52e95d6a9a431c1eeae6

Today I will work on surveying the API.

Regards

Pranav Garg