add an opportunity for item types to dynamically pick their dependencies
implement get_auto_deps for symlinks and directories
abuse get_auto_deps to detect collisions between files, symlinks and dirs

BundleError will be raised when:

file:/foo with directory:/foo/bar
file:/foo with symlink:/foo/bar
file:/foo with file:/foo/bar
file:/foo with directory:/foo
file:/foo with symlink:/foo
symlink:/foo with directory:/foo
directory:/foo with directory:/foo
symlink:/foo with symlink:/foo
file:/foo with file:/foo
use get_auto_deps for files to depend on symlinks and dirs as needed

note that this removes the static dep from file: to direcory:
directories, files and symlink must all depend on users now
duplicate definitions are now checked for all items

until now this was presented to the user as a dependency error, which might be hard to track down
Merge pull request #64 from trehn/issue-63

Automatic dependencies between files, symlinks and directories