Fix for Nginx 502 error: `upstream sent too big header while reading response header from upstream`
Merge remote-tracking branch 'simonwheatley/bugfix/502-response-headers'

Work toward PR# 135
Further document the fastcgi_* directives in nginx-wp-common.conf

Closes #135
Specify a charset of UTF8 in nginx-wp-common.conf
Document the try_files directive in nginx-wp-common.conf
Remove duplicate client_max_body_size directive from nginx-wp-common.conf

We already specify a client_max_body_size of 50M in nginx.conf. Having it in our common WP file as well makes it harder to overwrite the setting at an individual site level (I think).