Implementation of DBI interface

Support for:

• query preparing and numerical/string parameters binding
• transactions
• CLOBs (insertion)
• ORA-xxxxx errors handling
• NLS (tested for UTF-8 clients)

Extension to DBI API:

• Reusable queries for multiple execution of prepared query

Known bugs:

• a space is required before the question mark in parametrized SQL query
• reusable queries do not work for variable-length parameters, such as strings