Remove rel_path causes issues on windows across drives.

Given
	PWD is z:/src/gendev
	RUBY is installed in C:/Ruby193
	path1  = veewee/iso/sol-10-u11-ga-x86-dvd.iso
	path2  = Z:/src/gendev

When
	I run veewee vbox build <definition>
Then
	 I get the following error.

	C:/Ruby193/lib/ruby/1.9.1/pathname.rb:465:in `relative_path_from': different prefix: "" and "Z:/src/gendev" (ArgumentError)
	        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/veewee-0.3.12/lib/veewee/provider/core/helper/iso.rb:116:in `verify_iso'
	        from C:/Ruby193/lib/ruby/gems/1.9.1/gems/veewee-0.3.12/lib/veewee/provider/core/box/build.rb:37:in `build'

Since both a relative and absolute paths should work with curl, md5, and shasum there should be a need to validate a relative path to the iso.