Introduce prepared statements

[k00ni]

Prepared statements are a good way to counter SQL injections. After PDO was introduced (#109), we can transform ARC2 to provide prepared statements. Another advantage of this feature is speed up, if you use the same query multiple times.

• requires introduction of PDO - see Use adapter-approach to allow usage of mysqli, PDO, ... #109
• possible ways:
  • prepared statements on SQL level
  • prepared statements on SPARQL level

Any thoughts?