Add FluentEnumConverter, able to convert lower-cased, space-separated values to Enums

Copying&pasting from the javadoc:

An EnumConverter allowing stories prose to be more natural. Before performing the actual conversion, it transforms values to upper-case, with any non-alphanumeric character replaced by an underscore ('_').

Example: assuming we have defined the step "Given I am on the $page" which is mapped to the method iAmOnPage(PageEnum page), we can then write in a scenario:

Given I am on the login page

instead of:

Given I am on the LOGIN_PAGE

Warning. This requires enum constants to follow the standard conventions for constant names, i.e. all uppercase letters, with words separated by underscores.