Paypal form comment length and amount check (bug 1061203)

Enforce comment length to 255 characters max and the amount to be 0.01 min in addons.contribute view and form. (bug 1061203)
We try to have comments (in the code) that are as close as possible to real sentences, so that would be:
`# Enforce paypal-imposed comment length limit.`

(An uppercase letter to begin with, and a dot to end the sentence).