Bruteforce protection

This is a simple (but configurable) brute force protection for logins, it will block all IP addresses for 15 minutes who tried to login 10 times with invalid credentials within 15 minutes. Additionally, it will add an entry to the ownCloud log with the username + IP address.

I think this is enough, since I don't know anyone who enters his password wrong 10 times in 15 minutes ;-) - nevertheless, if one does this he will get a warning instead of the login screen saying that he should wait 15 minutes and then try it again.

Currently, it does not work with reverse proxies in place. This should get fixed before merging.

I'm not sure about the index in `db_structure.xml`, could anyone check if this is correct? THX.

Ref #1580 

\cc @jancborchardt @karlitschek