WebStat: clean real_escape_string for queries
- escape_string cleaned from webstat_engine.py, webstat.py bibsched.py and webbasket_migration_kit.py (fixes #44)
- Uses of escape_string are now replaced with new function real_escape_string in dbquery. This function properly escapes an unescaped string with regards to charset.
- Added function wash_table_column_name() to dbquery.py in miscutils. This function checks if the passed string is a valid table column name (alphanumeric).
- Appropriate test cases for new functions added in dbquery_tests.py.