Merge pull request #18 from chefkevin/create_text_type

feat(graphalchemy types): add Text type as String.class db type with no size limit.
feat(visualization generator): add structure and main classes.
feat(visualization generation): add configuration files.
feat(visualization generation): add graph title customization methods.
fix(visualization generation): fix configuration import.
feat(visualization generation): add test case for Graphviz generator..
feat(visualization generation): add Node centric graphviz code formatting method.
fix(visualization generation): correction of write method for node centric outputs.
fix(visualization generation): correction of node centric outputs filenaming.
fix(visualization generation)
fix(visualization generation): Get property name prefixes for visualization.
fix(visualization generation): ability to visualize multiple relations between two nodes.
fix(visualization generation): delete unwanted prints.
fix(visualization generation)
feat(ogm): moved documentation in class docstring.
fix(graphalchemy types)