Add th.text-center styling

[paroxp]

Started of with the addition to .text-center scss definition, as TH has not been considered there.

For maintainability purpose, I've refactored the way of these class declarations. I mean, with the previous approach, table.text-left, table.text-right, td.text-left and td.text-right were not even defined. My approach is to rule them all.

I've formatted the code, so it's clean, removed few blank lines, added spaces where necessary and removed the duplication of an attribute.

Hope that's fine.

Got confused with the contributing guide, so I've followed the one below:
https://github.com/zurb/foundation-emails/blob/develop/CONTRIBUTING.md#submitting-pull-requests

[rafibomb]

Love all these changes! Just one question: what is the need for the extra indentation in the button maps?

[paroxp]

Hi there,

Actually, this indentation has no particular reason, you're right.

I must have confused few bits and pieces, when IDE picked on it. Feel free to ignore it.

[rafibomb]

Love it - thanks!