Dependencies - #410

- Completed support for 'task 1 depends:2,-3' to manipulate the
  dependencies.
- Now supports rc.dependency.reminder to indicate when to nag about
  dependency chain violations, defaulting to on.
- Now supports rc.dependency.confirm to require confirmation before
  fixing dependency chains, defaulting to on.
- New source file dependency.cpp which implements a low-level API for
  determining dependency status, and assorted handlers for task state
  changes.
- Adds blocking tasks to the 'next' report.
- Added more dependency unit tests, changed the wording in a couple of
  them and numbered them for easy reference.

src/main.h

@@ -133,11 +133,11 @@ int handleExportiCal (std::string &);
 int handleExportYAML (std::string &);
 
 // dependency.cpp
-bool dependencyIsBlocked (Task&);
-void dependencyGetBlocked (Task&, std::vector <Task>&);
-bool dependencyIsBlocking (Task&);
-void dependencyGetBlocking (Task&, std::vector <Task>&);
-bool dependencyIsCircular (Task&);
+bool dependencyIsBlocked (const Task&);
+void dependencyGetBlocked (const Task&, std::vector <Task>&);
+bool dependencyIsBlocking (const Task&);
+void dependencyGetBlocking (const Task&, std::vector <Task>&);
+bool dependencyIsCircular (const Task&);
 void dependencyChainOnComplete (Task&);
 void dependencyChainOnStart (Task&);
 void dependencyChainOnModify (Task&, Task&);