Implemented person and webhook wrappers, generalized ResultsPageResponse.

* Initial implementation of all People methods.

* Initial implementation of all Webhook methods.

* Switched to a generalized <CommonResources/ResultsPageResponse> for pages of
  people, donations, webhooks, etc.

* Added RequestUrlBuilderAppendMethodNonNullParameters to
  <Session/NationBuilderHttpTransport> to reduce the code needed to pass the
  parameters of a wrapper method as URL query parameters with the same names.