Omit form-control class from file_field

The form-control class is not used for `<input type=file>` in the
Bootstrap examples. Adding this class introduces an undesirable
border to the choose file control.

This commit introduces an `:omit_control_class` option to the
general-purpose field builder that is `true` by default for
`file_field`.
Add :control_class option for per-field override.

This is a cleaner solution to allowing for special behavior needed
for file_field, as suggested by @potenza.
Merge pull request #49 from mbrictson/omit-control-class

Omit form-control class from file_field