disable escape_string_warning option 

With Joomla 3.0.1, using Postgresql 9.1, the following warning appears in PostgreSQL logs many many times:

WARNING: nonstandard use of \\ in a string literal at character 187
HINT: Use the escape string syntax for backslashes, e.g., E'\\'.

These warnings appears because Joomla use backslashes as escape characters (ie standard_conforming_strings is disabled).

This PR disables "escape_string_warning" option. This option is supported since PostgreSQL 8.1