Correct the default Logback config location in the documentation

Fixes #1247
Ensure custom HTTP code mappings for /health don't remove default mappings

Previously any custom http code mapping would remove the default mappings. With this commit the behaviour is changed so that default mappings will stay if a custom mapping is registered. Certainly a default mapping can be overridden.

fixes #1264
Improve performance on JVM 7

Detects presence of `getClassLoadingLock` method and uses it when
available. In case of JVM 6 uses plain old synchronization on `this`.

This patch uses assumption that spring-boot compiles on JDK >= 7.