fixed: csrf token is const

[cuongth]

Thanks for Nicolay Garanko's explanation, I can understand his implementation (boss_csrf_filter.erl) and can use it smoothly.
He also provides simple app to test boss_form and boss_csrf_filter: github.com/ngaranko/r2d2

I recognized that csrf_token doesn't change after refresh /public/login, even delete cookies.
The problem is random:uniform(64) returns constant.
Prefer: http://pdincau.wordpress.com/2012/02/15/how-to-generate-random-numbers-in-erlang/