implement sum helper as strict alias for add helper

[Mifrill]

closes #40

[Mifrill]

Hi, @skeate back to your #40 (comment)

I still don't agree with adding a separate sum helper, especially if it's not just an alias to add. Having to check the docs all the time to remember which was which would be a pain.

I did a bit confused in fact that the documentation for the addon math does not include sum helper for a quite simple case like get the sum of two numbers, you know. After some search, I did find your #40 (comment) to realized that it exists actually.
I assume not only me:
https://github.com/search?q=ember+%22export+function+sum%22&type=Code
Even though the name add is a quite correct naming for the underneath logic, the name sum sounds more native from a human perspective.
I think it's nice to highlight it and have sum helper. It seems like, it's more about consistent naming, so what do you think about add strict alias?

[RobbieTheWagner]

@Mifrill I think we could just export from add here and be good to go. Something like this maybe?

Suggested change

	export { default, sum } from 'ember-math-helpers/helpers/sum';
	export { default, add as sum } from 'ember-math-helpers/helpers/add';

[Mifrill]

@rwwagner90 well, in this way file addon/helpers/sum.js will came redundant, right? So, then we would need to customize documentation somehow that provided by ember-cli-addon-docs because comments in file addon/helpers/sum.js have auto-converts into the component with documentation.
However, the original proposal is to have the documented sum helper:

maybe it's better to keep it consistent with the documentation addon with no this requested change, WDYT?

[Mifrill]

@rwwagner90 could you give an advice on how to customize documentation for this particular case?