fixed charset detection with wrong regexp.

I asume that you try to find the standard «# -*- coding: utf-8 -*-» expression at the start of the readed file, but you simply use the wrong regexp since you happily re-search with `_line_re` instead of `_coding_re`.

with django, I have a file that start with «try:».

your code try to find the charset. found «try:». 
so it continue until

        try:
            codecs.lookup(charset)
        except LookupError:
            charset = 'utf-8'

and mysteriously, it seem that a «try:» codding existe in python since `codecs.lookup` dont raise with this wrong charset.

and finally the declared charset become «try:»...

and of cours it crash whene trying to open with this charset.