Update and fix place_simple_hold()

This commit makes the following changes to the place_simple_hold()
subroutine:
 - clarify that the patron parameter is an ID
 - use a more typical default location for the oils_header.pl file
 - use a Fieldmapper object rather than raw JSON to construct the
hold request
 - add freeze/thaw to hold process to prevent custom status issues
 - add comments where customization is likely needed

We should probably revisit the freeze/thaw process in the future,
but this at least gets us to a working state.

Signed-off-by: Dan Wells <dbw2@calvin.edu>