Oracle fixes

This PR adds missing Backticks, replaces LIMIT with the correct parameter, adds debug output when errors occur, fixes oracle connection establishing, allows installation when only oracle is available and adds a `to_char` workaround for CLOB columns.

To test try the shiny vm at https://s3.owncloud.com/owncloud/public.php?service=files&t=d87645d07b443c4591c5a8aea77d94e2

Nevermind that the filecache is not working, thats beeing discussed in https://github.com/owncloud/core/pull/2389