Fix table creation

[andrykonchin]

Changes:

• Fix issue with using :date/:datetime as a partition/sort key and storing with string format
• Fix handling field type of custom type specified with dynamoid_field_type - it was used as native DynamoDb attribute type but should be treated as Dynamoid field type
• Use new proper exception Dynamoid::Errors::UnsupportedKeyType

The main issue: :date and :datetime fields can be stored in string format but when table is created Number type is used unconditionally.

[coveralls]

Coverage increased (+0.1%) to 98.564% when pulling 9b23ae7 on fix-table-creation into 2744fa5 on master.

[pboling]

LGTM