Generate warning for Cascading field names when naming steps

[kkrugler]

We know that using anything other than valid Java identifier names can cause problems with ExpressionFunction & ExpressionFilter, due to limitations in what the Janino compiler can infer from a text snippet. In addition, names shouldn't start with an uppercase letter.

So it would be nice to have a way to generate warnings when problematic field names were used. This ideally would be something you could control via a parameter passed to FlowConnector.connect(), but this would be a start.

[kkrugler]

So to clarify, I'm thinking of adding code to the step naming support already in Cascading utils. Though I could create an abstract BaseConnector that provides this support, and would be returned by HadoopPlatform/LocalPlatform.