* ext/digest/digest.c:
  - remove HMAC from list of digest algorithms,
  - add MD5 in list of digest algorithms,
  - add information about writing a C digest implementation using
    Digest::Base,
  - add documentation for Digest::Base public methods.

* ext/digest/md5/md5init.c: add examples for MD5.

* ext/digest/rmd160/rmd160init.c: add examples for Digest::RMD160.

* ext/digest/sha1/sha1init.c: add examples for Digest::SHA1.