EnumValueName check implemented

This check should force a specific formatting of  enum values. As of a discussion for what we should call constants and what - static objects, the suggested check provides an ability to specify format for them differently. Distinguishing (discussed in https://github.com/checkstyle/checkstyle/issues/23#issuecomment-26578088) is made on the basis of user-defined methods existing in the enum class.

Unfortunately, I could not find any references to xdocs, that is why they are absent here at all.