[Fix #265]: Remove `font-size` rules from .pure-form children

This commit removes `font-size` rules from `<input>`, `<legend>`, and
`<fieldset>` elements within `.pure-form`. I did keep the font-size
rules for `.pure-form-help-inline` since that size is meant to denote
importance.
add HISTORY.md entry for #265 fix
update font-sizes to be whole-number px values when converted
Merge pull request #294 from tilomitra/forms-font-size

[Fix #265]: Remove `font-size` rules from .pure-form children