add back common rules for all grid units
Fix #41 by introducing a .pure-fr class

The .pure-fr class stands for "font-reset". If someone is using a font
that is substantially wider than Arial/Helvetica, the grid may break
for them (issue #41). They can stamp their `.pure-g` with the `.pure-fr`
class, and add their font-family in with this:

```
/* all child elements of .pure-fr get a custom font */
.pure-fr [class *= "pure-u"] { font-family: "Foo", sans-serif; }
```
add test for font checking and IE7 breakage bug (#154)