Fix HTML markup to (almost) valid HTML

* `index.htm`: fix stray end tags.
* `index.htm`: fix HTML entities.
* `index.htm`: remove cellspacing, border, and cellpadding attributes.
* `css/base.css`: added table styling to the css.
Added Dutch locale
- version 1.5.5
Merge branch 'fix-html' of https://github.com/timothymctim/GoodCalc into minor-fixes
Merge branch 'nl-locale' of https://github.com/timothymctim/GoodCalc into minor-fixes
Merge branch 'minor-fixes'
- 1.6 version