Add builders for primitive & group types

[sunchao]

Currently with new_primitive_type(..) and new_group_type(..), we always have to pass all the parameters, even though some of them are usually default (e.g., precision, scale, length).

This adds builders for constructing PrimitiveType and GroupType, which start with a default set of attributes and can append more with with_XX(..) methods.

[sunchao]

@sadikovi would you mind reviewing this one?

[coveralls]

Coverage increased (+0.1%) to 85.665% when pulling ca50f72c7e69c59f5a92b361a506f1e6d1b47ec8 on type_builder into c8eb3dd6b65cd4b519002d2c67269823b110f3d4 on master.

[sunchao]

Thanks. Merged.