Adding make_column_transformer

[wlandecker]

Gave this issue a shot: https://github.com/jnothman/searchgrid/issues/13

ColumnTransformer breaks from Pipeline and FeatureUnion's pattern by accepting a list of (encoder, 'column_name') tuples, instead of just a list of encoders. As a result, one question for this PR is whether searchgrid should pass a transformer name to ColumnTransformer.transformer that only contains the encoder class name, or if it should also include the column name. To be consistent with make_pipeline and make_union, I chose only the encoder class name.

@jnothman I'd love any comments or suggestions!

[coveralls]

Coverage increased (+0.1%) to 99.115% when pulling 3d5c30dde6ad142ec6bb0203f8b19fa2bb81bb48 on wlandecker:add_column_transformer into 2c25bce361c7a3868a25cab7aa38f2762519e94a on jnothman:master.

[wlandecker]

~~Oh shoot, I just realized that in ColumnTransformer, column_name is expected to be a list of column name strings, not a string itself. Let me fix that first...~~

Fixed.

[jnothman]

My time is pretty limited at the moment but I really appreciate your interest and help and will try to give this a careful look soon

[wlandecker]

No worries! Take your time.