Custom preseed file support

Preseed file may be specified like this:
    PRESEED=/path/to/my-preseed.cfg ./build.sh
Document custom preseed
Custom late_command.sh support
Optional VM GUI mode
Optional VM GUI mode
Ignore vi swap files
Fix typo
Merge branch 'custom-preseed' of github.com:timka/vagrant-debian-wheezy-64 into timka-extras
Cleanup build.sh and readme