Split -vendor-placeholder selector by vendor.

Some browsers (specifically Firefox(18) that I know of) fail to select
correctly if the selectors are combined. This was fixed in bootstrap
(https://github.com/twitter/bootstrap/blob/master/less/forms.less#L210).
Not sure of the exact commit but there is a nice comment.
Merge pull request #39 from bendoerr/patch-1

Split -vendor-placeholder selector by vendor.