Samba 4.0.7 / iOS SDK 7.0

Added `libtevent.a` to the project to fulfill Samba 4.0.7 depencies.
Changed Samba version in `Rakefile` to last the last one.
Changed the `SAMBA_BASE_URL` to keep getting the correct file even if
the last version changes.
Added `libtevent` to `SMB_LIBS`.