Add transform module content function to be used in postgres

For example,

`SELECT html_content(encode(file, 'escape')::text) FROM files`

to get the cnxml in "file" transformed to html,
but this does not add any files to the database.

MANUAL-UPGRADE: add a function (see schema.sql) to existing db
Merge pull request #226 from Connexions/transform-module-content

Add transform module content function to be used in postgres