adjust logic-fetching sql for logic-graph-nodes

- Use With-Clause instead of temporary
  table (Select Into).
remove mysql-gem from Gemfile

- We actually use PostgreSQL now, so
  we don't need the mysql-gem anymore.
Extract graph-data fetching into own class

- Since we can reuse the idea for fetching
  the graphs nodes in ontologies, this
  extracts the functionality into
  its own flexible class.
adjust GraphDataFetcher to fetch nodes and edges.