Fix checkbox contrast

In user research we noticed that our checkboxes are rendering oddly on ios8

![screen shot 2014-11-11 at 17 30 18](https://cloud.githubusercontent.com/assets/68009/4998209/324f6d02-69ce-11e4-87a4-715f473063b0.png)

Investigation reveals that we are actually overriding the border, width, and background colour of the checkbox. These rules aren't respected by other browsers but they are in ios8 safari.

I've tightened up the scope of the styling on input (which I should have done from the outset anyway, really) and i've removed the unnecessary overrides on the checkbox inputs too.

This PR also removes some bits of CSS that didn't need to be there (the li, and @core-14/16)

I've had to use a class to more tightly scope the input, so that it works in ie6. Obviously, it's nicer to use input[type=text] but ie6 doesn't respect that. I'm interested if anyone has any nicer ways to do this, adding class=text to stuff seems a bit gross.

Here is the after shot:

![screen shot 2014-11-11 at 18 20 12](https://cloud.githubusercontent.com/assets/68009/4998345/66c6e47e-69cf-11e4-803c-66729dd823e5.png)