Make .btn-xs smaller and consider .input-xs for use inside tables

Created by: stev47

I'd like to propose changing .btn-xs, making it smaller to fit inside a table-row (i.e. not causing any increase in height of the table-row). Because the size difference between .btn-sm and .btn-xs is currently hardly noticable, it should be no problem to make .btn-xs a little bit smaller.

The addition of an .input-xs class to the existing .input-sm, .input-lg classes would also be nice for use inside tables and for consistency with .btn-xs, .btn-sm and .btn-lg.