Documentation that the "monad" wrapper requires user-defined alexEOF
Helpers for extracting user state in "monadUserState" and documentation.

Added functions "alexGetUserState" and "alexSetUserState" to the
monadUserState wrapper which interact with the user state field, to make
setting/extracting state more direct.

Updated the examples in the documentation to use these.
Merge pull request #23 from tlawson/master

Suggested updates to monadUserState and documentation