Add validation functionality to individual Param

This issue was brought up by https://issues.apache.org/jira/browse/SPARK-7147: There are multiple locations in which a parameter can be specified (in setters and in calling fit/transform), and we should check the parameter value in each one. It would be nice to avoid duplicating the checking code.

Proposed change:

• Add field to class Param: val isValid: T => Boolean, with default value of always returning true
• Add helper methods in new ParamCheck object for constructing common validation checks (especially to help Java users)
• Ensure that this validation is always done, most likely by always settings parameters by constructing a ParamPair + validating inside the ParamPair constructor