Cleaned up warnings for compiling Blink.pde on all boards

*** Please do not merge this pull request yet.  It needs a thorough going over. ***

I have made the example sketch Blink.pde compile cleanly without warnings (save the erroneous .comment section warning) on all boards.  The included patch to the boards.txt enables all warnings, so may not be wanted when it comes time to merge.

Most of the changes were unused variables, or explicit casting needed.  There were a few strange ones that could do with sanity checking.

Please go over the changes and add comments.