Support for authentication using external proxy

[jakubman1]

Simplified version of #32 that uses external proxy instead of implementing authentication directly in ExaFS.

Makes use of proxy such as Apache that handles the authentication and fills the specified HTTP header.

Example of Apache configuration for database authentication:

# mod_dbd configuration
DBDriver pgsql
DBDParams "dbname=exafs_users host=localhost user=exafs password=verysecurepassword"

DBDMin  4
DBDKeep 8
DBDMax  20
DBDExptime 300

# ExaFS authentication
<VirtualHost *:80>
    ServerName example.com
    DocumentRoot /var/www/html

    <Location />
    AuthType Basic
    AuthName "Database Authentication"
    AuthBasicProvider dbd
    AuthDBDUserPWQuery "SELECT pass_hash AS password FROM \"users\" WHERE email = %s"
    Require valid-user
    RequestHeader set X-Authenticated-User expr=%{REMOTE_USER}
    ProxyPass http://127.0.0.1:8080/
    </Location>
</VirtualHost>

[jirivrany]

Good work, thank you.