Update README, add docstrings and add current-language-name function

Updated the README to match the new API, added docstrings to all public functions, to class slots and packages.

Added the `inflector` package nickname. Inspired by Edi Weitz practice. Just a minor change, feel free to reject it.

Added the `current-language-name` function. In my opinion, the `current-language` function shouldn't be exported: it returns the language object instead of the language name; it is only used by internal functions; and the methods to handle the language object aren't exported. 

Now, if the user wants to know the current language name, he must write `(cl-inflector.langs::name (current-language))`.