Add reduce validator; gravatar_for update

Added reduce_validator so that the sign-up page only shows relevant
errors (i.e. No password is typed in, so only the “Password can’t be
blank” error is shown, out of all of the password errors).

Added the ability to change the default gravatar size by inserting <%=
gravatar_for(@user, size: xx %>.
Merge branch 'sign-up' of https://github.com/junebug217/sample_app into sign-up