Add ability to detect spec version optionally

[valfirst]

There could be cases when schemas are managed by third-parties, in this case client side logic can either use spec version defined in schema if available or some default one. Having current implementation of SpecVersionDetector it's not really convenient to catch exception to fallback to some default value then. Also exceptions are quite heavy mechanism, thus new method is proposed which returns an empty Optional value, if the schema tag is not present.

Also small refactoring of SpecVersionDetector class is done in order to make it true-utility class:

• class is marked as final
• private constructor is added

[stevehu]

@valfirst Thanks a lot for your help.