More functionality for model attributes

[khotilov]

• Learner and C-api: added a method to retrieve the names of available attributes
• Learner and C-api: added ability to delete an attribute (C-api uses the "setting null value means delete" idiom; Learner gets a separate method for delete, since I didn't want to change its setter interface)
• R: getters and setters for xgboost attributes
• R: separate function to set xgboost parameters
• R: in xgb.Booster constructor, when both modelfile and params are provided, set params after loading from modelfile
• python: added attr_names() method; calling set_attr(a = Null) would delete an attribute

Let me know if "setting null value means delete" is not acceptable for C or Python interface. I can add separate attr deletion methods.

[tqchen]

how can it become list of strings?

[tqchen]

no problem, I mistunderstand the function from_cstr_to_pystr

[tqchen]

Thanks for the changes. It LGTM. We might want to make python API consistent with R's, i.e.

remove get_attr_names, but directly provides a attributes function that returns a dict of name->value

[khotilov]

Done.

Note that Python's attributes() returns {} when no attributes while in R it would return NULL. In R I did it so it acts similar to R's native attributes() function. However, I feel that in Python it is more natural to use empty [] or {}, as it usually saves checking for None. It you want it to return None - no problem - I can change to that.

[tqchen]

This sounds good to me, thanks!