[WIP] Substitute rewrite with proxypass.

This MR aims to substitute the rewrite rule with proxypass.

@mosmani how should the location block for root look like?

Closes #63

@mosmani please see question on description and review.

@balameb here is the complete vhost, you dont need a location definition:

### set in gitlab.rb
# nginx
#external_url 'https://gitlab.example.com'
#nginx['enable'] = false
#
# workhorse
#gitlab_workhorse['listen_network'] = "tcp"
#gitlab_workhorse['listen_addr'] = "127.0.0.1:8181"
#web_server['external_users'] = ['www-data']
#
# rails app
#gitlab_rails['internal_api_url'] = 'https://gitlab.example.com'
#gitlab_rails['time_zone'] = 'Europe/Berlin'

<VirtualHost *:80>
    ServerName gitlab.example.com

    Redirect permanent "/" "https://gitlab.example.com/"
</VirtualHost>

<IfModule mod_ssl.c>
<VirtualHost *:443>
    ServerName gitlab.example.com

    ProxyRequests Off
    SSLProxyEngine On
    ProxyPreserveHost On
    AllowEncodedSlashes NoDecode

    ProxyPass / http://127.0.0.1:8181/ keepalive=On connectiontimeout=10 retry=0
    ProxyPassReverse / http://127.0.0.1:8181/

    RequestHeader set X_FORWARDED_PROTO 'https'
    RequestHeader set X-Forwarded-Ssl on

    SSLEngine on
    SSLCertificateFile    /etc/apache2/ssl.crt/gitlab.example.com.crt
    SSLCertificateKeyFile /etc/apache2/ssl.key/gitlab.example.com.key

    ErrorLog  ${APACHE_LOG_DIR}/gitlab.example.com-error.log
    CustomLog ${APACHE_LOG_DIR}/gitlab.example.com-access.log combined
</VirtualHost>
</IfModule>

@mosmani right but what about other options such as SSLProtocol all -SSLv2or the ErrorDocument options? Don't we want to include those as well?

When you start using mod_proxy with "/" then every other directive concerning conent (ErrorDocument) is useless because every request will be routed through the proxy to the backend.

From my point of view every person who calls him "admin" should care about the SSL/TLS Security of his server in general and not only for one vhost thats why i didnt insert all SSL/TLS options in my generic vhost suggestion.