Add user_agent_parser as executable in gemspec.
Do not add lib to load path in bin/user_agent_parser.

The method File#readlink does only work for symlink, hence if the binary
is not symlinked, it will fail.

Adding the lib directory to load path did also not do anything
different as it worked when removed.
Add information about user_agent_parser to Readme.
Version 2.1.4