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`.
Xcode 4 rollback

Rolling back to SDK 6.1, waiting for a final version of Xcode 5.
`XCODE_PATH` now is retrieved from `xcode-select`.
smbc Debug

Added a constant for Debug Level.
KxSMBSample improvements

Alert view added to specify host.
Button to connect to other host.
Last server used is saved in the user preferences and autocompleted
when the app is loaded.
Back buttons now shows only the folder name instead of the full path.
TreeViewController is now a UITableViewController.
Refresh control added.
TreeViewController updates file list on path change.
Head TreeVC is now reloaded when the authentication has been done.
Image preview in FileViewController.
Merge pull request #7 from gonzalolarralde/master

KxSMB now compiling with last version of Samba, and sample project improvements