performance improvement - ~AssertionHandler, getConfig return value

inlined ~AssertionHandler(), and changed getConfig to return
shared_ptr const reference instead of by value (very expensive for
shared_ptr's).

Further work on #1086.

Brings test from 0m20.635s to 0m17.186s

include/internal/catch_context.cpp

@@ -20,7 +20,7 @@ namespace Catch {
             return m_runner;
         }
 
-        virtual IConfigPtr getConfig() const override {
+        virtual IConfigPtr const & getConfig() const override {
             return m_config;
         }