Raise a warning about the inefficacy of allow_blank
Raise a warning about the inefficacy of allow_blank

Closes #24

subyraman/wtforms,skytreader/wtforms,cklein/wtforms,Aaron1992/wtforms,pawl/wtforms,Xender/wtforms,hsum/wtforms,pawl/wtforms,jmagnusson/wtforms,Aaron1992/wtforms,wtforms/wtforms,crast/wtforms