Command Line Parsing

- Implemented Arguments::extract_command to locate the command
  keyword in an argument list.
- Implemented Arguments::extract_sequence to locate and remove an ID
  sequence from an argument list.
- Added unit test for extract_sequence.
- Stubbed all Argument::extract_<object> methods.
- Simplified code in (soon to be obsolete) Sequence.cpp.

src/main.cpp

@@ -63,7 +63,7 @@ int main (int argc, char** argv)
 
   try
   {
-    context.initialize (argc, argv);
+    context.initialize (argc, (const char**)argv);
     status = context.run ();
   }