Add support for classes to isometric objects.

[joelburget]

Fixes #216

[elchininet]

Hi @joelburget,
Thanks for contributing 🙂
Some things that I have observed in a quick look, please, fix them and I'll review the code with more attention:

1. The className should not have an empty string as a default value, this parameter should be added only if it has a value. The snapshots that are not using that option should not have an empty class parameter.
2. There should be tests cases for this new option
3. This new option should be added to the documentation to each class

Regards and thanks again for contributing to the project.

[joelburget]

I think I fulfilled all of your requests, please let me know how it looks.

[elchininet]

I think I fulfilled all of your requests, please let me know how it looks.

@joelburget, looks good at first sight. I'll take a deeper look when I have some time. 👍

[elchininet]

LGTM, just some removals are required.

[elchininet]

@joelburget,
It is ready to merge, rebase with master and solve the conflicts with dist/web/isometric.js.

[joelburget]

I just merged and rebuilt dist/web/isometric.js (git p --no-rebase upstream master; npm run build; git add dist/web/isometric.js)

[coveralls]

coverage: 100.0%. remained the same
when pulling 0b7554c on joelburget:classes
into defa492 on elchininet:master.