Automated gemspec and method definition
- automated the section of files list in .gemspec file
  by using `git ls-files lib` command (remember to add new
  files to git, before building the gem);
- defined 'method_missing' method for some classes, that are used as
  data structure classes, in order to automate methods definition
  according to the data passed to class constructor;
- implemented the functionality that allows to retrieve player's
  achievements and if necessary, to get details about the specific
  achievement;
- implemented the functionality that allows to retrieve player's
  vehicle and if necessary, to get details about the specific tank.
- extended the parser by defining new functions;
- defined the Wot::Api function, that caches achievements list after
  the first request, and then returns previously parsed achievement list.