Simplify communication with DB
- use of MyBatis framework
#24 throws exceptions early, catch them late
- minor fix (possibility to work without db)
- throw own exception and handle it with error page
fix #20: diagram can be downloaded as raw JSON
re #20: use icomoon icons
re #20: add possibility to download diagram as JSON
#23 extend DB column for saving diagram JSON
- updated DB graph_json datatype
detach value objects to separate module
multi-module maven project
move plugins to sources directory