Add the POST /store/coupon and GET /store/coupon/create handlers.

[cdepillabout]

If you run the api, and then access http://localhost/store/coupon/create you will be able to create a new coupon.

This implements two of the handlers from #77.

@arowM Please review.

[arowM]

@cdepillabout
Thanks for renaming to readable variable name and fixing specification documents.

When I tried to create a coupon with validFrom on Chrome, the page show me an error bellow.

could not parse: `2016-12-14'

Even if I do not specify validFrom and validUntil, the page show me an error bellow.

could not parse: `'

Also, I've created extra issue related to this PR on #83 and #85.

[arowM]

The "30" I injected is just a sample.
If you do not have explicit reason to use "30" as a default value, please use mempty as default.
(Please fix line 117, 144, 153 as well as this line)

[cdepillabout]

This is fixed in commit d289dfb.

[cdepillabout]

@arowM

When I tried to create a coupon with validFrom on Chrome, the page show me an error bellow.

could not parse: `2016-12-14'

This has been fixed in commit ced7968.

Even if I do not specify validFrom and validUntil, the page show me an error bellow.

could not parse: `'

This has been fixed in commit 87b579c.

[cdepillabout]

I'll go ahead and merge this in.