Introduce Active Directory support.
Thanks to Tylor Winfield! I adopted the configure section layout
and some some other little things from https://github.com/jamf/NetSUS/pull/14

This implementation doesn't need an proxy user for authentication.
Things you have to setup are stated in LDAP settings
- searchdn
- domainname
- admin groups. yes. only groups. (but nested is allowed ofc)

added files:
images/settings/ldap.png:
as provided by Tylor Winfield - came with jss some versions ago I guess

inc/auth.activedirectory.php:
an authentication class for activedirectory. may works with other ldap's too.

ldap.php:
settings page, slightly changed but mainly from Tylor Winfield

accounts.php:
ldap users don't have to enter "webadmins" password to change it.
(they even can't enter it anyway now).
webadmin itself still has to enter the current password to change it.

inc/config.php:
adding ldap functions for storing and retrieving out of the config.
get and set filter for their configureable variables

inc/functions.php:
we return the correct user now. since it alway return the webadmin's name
for active directory this wouldn't have been correct.
The current name is stored inside it's session

index.php:
store the username which authenticates within the session.
add active directory as "fallback" authentication.

settings.php:
see Tyler Winfields commit - add LDAP settings.

If you find any bug, please report it or pull request :)