Octave: add support for version 4.2

- .travis.yml:
  - ppa:kwwette/octaves has Octave version 4.2, also run C++11 tests
- configure.ac:
  - prefer Octave program "octave-cli" to "octave"
  - extract any -std=* flags from CXX, add to OCTAVE_CXXFLAGS
- Lib/typemaps/fragments.swg:
  - SWIG_isfinite_func(): extern "C++" is required as this fragment can
    end up inside an extern "C" { } block
- Lib/octave:
  - add std_wstring.i (copied from std_string.i) for C++11 tests
- Lib/octave/octrun.swg:
  - move Octave version-checking macros to octruntime.swg
  - Octave single()/double() functions now call .as_single()/.as_double()
    methods; redirect calls to __float__() method as per .scalar_value()
  - << and >> operators are no longer supported by Octave
- Lib/octave/octruntime.swg:
  - move Octave version-checking macros here for conditional #includes
  - #include interpreter.h instead of #toplev.h
  - #include call-stack.h (now needed for octave_call_stack)
  - unwind_protect is now in octave:: namespace
  - error_state and warning_state are deprecated; use try/catch to catch
    errors in feval() instead
  - always set octave_exit = ::_Exit, to try to prevent segfault on exit
- Lib/octave/octopers.swg:
  - << and >> operators are no longer supported by Octave
- Lib/octave/exception.i:
  - Add macro SWIG_RETHROW_OCTAVE_EXCEPTIONS which rethrows any
    exceptions raised by Octave >= 4.2
- Examples/test-suite/exception_order.i:
  - Use macro SWIG_RETHROW_OCTAVE_EXCEPTIONS to rethrow exceptions
    raised by error() function in Octave >= 4.2
- Update Doc/Manual/Octave.html and CHANGES.current

Examples/test-suite/exception_order.i

@@ -23,7 +23,17 @@
    user's throw declarations.
 */
 
-#if defined(SWIGUTL)
+#if defined(SWIGOCTAVE)
+%exception {
+  try {
+    $action
+  }
+  SWIG_RETHROW_OCTAVE_EXCEPTIONS
+  catch(...) {
+    SWIG_exception(SWIG_RuntimeError,"postcatch unknown");
+  }
+}
+#elif defined(SWIGUTL)
 %exception {
   try {
     $action