Bring `lead` styles more inline with GOV.UK ones

Bootstrap uses a lighter font weight for text using the lead class. This lighter text is harder to read and looks out of place with all of GOV.UK’s other font rendering.

* Reset back to normal font weight
* Lighten text colour to maintain balance

## Before
![screen shot 2014-09-05 at 14 13 13](https://cloud.githubusercontent.com/assets/319055/4165090/9de9137e-34fe-11e4-9dca-5d021f5a5547.png)

## After
![screen shot 2014-09-05 at 14 12 56](https://cloud.githubusercontent.com/assets/319055/4165089/9dd33f72-34fe-11e4-94a8-6c21e54a9d0d.png)