Move Phinx-type references to interface constants

By moving the Phinx types to constants, code within the adapters is more readable. The adapter-specific column type strings are clearly distinguishable from the abstract Phinx types (both were represented as similarly formatted strings). As well, having the tool's supported types collected together in the AdapterInterface helps in understanding the code's architecture, and will facilitate developing new adapters.
Merge pull request #220 from cyrusboadway/feature/refactor-types-to-constants

Move Phinx-type references to interface constants