Pipeline reorg

Major reorganization of Heka pipeline and plugin implementation code. No functional or behavioral differences at all. As few API changes as possible, limited to a) movement of code from one package to another, b) occasional changes in a variable's scope to support access from outside the package, c) occasional introduction of getter function to support access from outside the package, and d) occasional function rename to clarify behavior based on code's new context.

For code review, I recommend browsing through the pipeline (http://is.gd/36f32U) and plugins (http://is.gd/42r07C) folders on this branch in addition to reviewing the diffs, to get an overview of how things organized. Resolves #541.