support for custom column types

[nRo]

introduce a value type abstraction to support custom column types (like temporal column types #21 )
Value types should provide the following methods:

• read/write values to DataOutputStream
• convert values to String
• parse values from String

These value types must then be used in the following library parts:

• CSV reader / writer
• Automatic column type detection
• Printer
• Filter query parser

Value types are currently being worked on in branch
value-type-abstraction

Progress:

• there are value types for all previously available columns.
• value type object are available from column, row and header objects
• CSV writer uses value types to write String representations

[nRo]

the feature branch is now merged into master