Adding .AndDoes() option to Returns() and cleaning up SubState

- Returns() and co. now return ConfiguredCall objs, so additional
  call configs can be made like .AndDoes(...) for callbacks.
- Removing properties in SubstituteState that are actually stateless
  dependencies.
- Removing CallSpecFactory creation cruft from SubstituteState
  Yes, a factory factory. No, I'm not proud.
- Renamed IResultSetter to IConfigureCall
  Handles configuring results and actions for calls/call specs.
- Splitting call spec retrieval logic out from ConfigureCall to new class.
  GetCallSpec now holds logic of how to get call spec from last call and
  pending call spec.
XML comments for public methods

in SubstituteExtensions
Playing around with the wording of xmldoc comments.
Updating documentation to include AndDoes.
Adding documentation for checking event subscriptions (#107)
Updating changelog for 1.6.0 release
Updating site for 1.6.0 release distribution
Updating nuget.exe.
Linking to relevant section of return-from-function page from callbacks page
Add pending test for detecting reentrant sub calls in Returns