Set Avatar for LDAP users automatically (if a picture is available)

Upon first login (and then regularly once a day) the profile picture (= jpegPhoto attribute) from LDAP is fetched, adjusted and set as User Avatar. It allow LDAP users to change their avatar only, when a picture in LDAP is not available, otherwise it is blocked. 

This required additions in User Management.

Also Avatar is extended to accept an OC_Image as parameter instead of only image data or file.

Local users are not affected.

This is how Personal page looks like for an LDAP user with photo set (controlled by LDAP):

![ldap user with pic](https://f.cloud.github.com/assets/2184312/1621660/23349ce2-5693-11e3-9858-69dbe923e68f.png)

and this without (still controlled by user):

![ldap user without pic](https://f.cloud.github.com/assets/2184312/1621663/3688a180-5693-11e3-9df0-735cec84d2fc.png)

(In the second scenario, when the user sets a picture it is not stored in LDAP. We do not write to LDAP).

@jancborchardt The message on the first screenshot is ok?

Please review @karlitschek @icewind1991 @Kondou-ger @PVince81 or others please.