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
Added windows check for md5 command
changed -d switch to --make-directories since it was complaining on windows
added quotes around MKISOFS call since there can't be spaces in paths
updated README.md with windows tools
Moved windows instructions below linux
Fixed README formating
Update cpio to be compatible with Windows
Update for Debian 7.4
Ensure ISO is unmounted after installation
Merge pull request #25 from cliffrowley/master

Update for Debian 7.4
Reduce size of created box file from 540MB down to 356MB