version 2.3

Fixes:

• Updated sqlite version to 3.12.0
• Added forgotten operator << overload #46
• Added support for nullptr and uniqueptr<T> and deprecated boost::optional. #52
• Fixed exceptions terminating the program #53

You can find more info in the updated README file.

Breaking changes:

• prepared statements are not uniqueptr<database_binder> anymore, they are plain database_binder instances.
  so instead of prepared_statment->reset() you should write prepared_statement.reset().
• boost_optional<T> is deprecated, we will replace this with std::experimental::optional<T> when enough compilers support it.