Unify processor interface. Check boundaries while processing. Fixes #205

This commit unifies the interface of all `SectionProcessorBase` `process...` methods.
Previously, it was expected that `processSignature()`, `processDescription()` and `processContent()`
always processes exactly one node (or none). However with the Blueprint `SectionProcessor` processing (arguably not correctly)
multiple nodes it is clear that is essential to expect any `process...` methods to process an arbitrary number of nodes. For this
the interface of aforementioned nodes needed to be extended to allow checking the boundary of the node iterator.
Merge pull request #206 from apiaryio/zdne/uniform-processor-interface

Unify processor interface. Check boundaries while processing. Fixes #205