Added signal handlers (and placeholder for SEH handlers)

- based on PR 232 (https://github.com/philsquared/Catch/pull/232 - thanks Lukasz Forynski)
- Writes to reporter, so gets all the usual context, but then exits directly (since the stack cannot be resumed) so no summary
- On Windows does nothing, as yet.

include/internal/catch_interfaces_capture.h

@@ -21,6 +21,7 @@ namespace Catch {
     struct MessageInfo;
     class ScopedMessageBuilder;
     struct Counts;
+    class ResultBuilder;
 
     struct IResultCapture {
 
@@ -35,6 +36,8 @@ namespace Catch {
 
         virtual std::string getCurrentTestName() const = 0;
         virtual const AssertionResult* getLastResult() const = 0;
+
+        virtual ResultBuilder makeUnexpectedResultBuilder() const = 0;
     };
 
     IResultCapture& getResultCapture();