Issue #30: Improve type handling

[codeforkjeff]

Resolves issue #30

• The convert functions in SQLiteAdapter now all return recognized affinities in SQLite.
• get_columns_in_relation() used to report a column's type as TEXT if SQLite didn't report any type affinity for the column. I think this was in place to make incremental models work. I changed this to UNKNOWN because that's less misleading. This is what will show up for column types in dbt-generated docs.
• Add notes to test_data_types about how types work in SQLite and do the best we can in those tests.