handle if box path contains space character

if I use box from my local computer that placed inner folder with name contains space characters, this error showing up

```
novalagung@dreambender:~/Documents$ vagrant box add some\ path\ \ with\ space/ubuntu1204.box

/opt/vagrant/embedded/lib/ruby/2.0.0/uri/common.rb:176:in `split': bad URI(is not URI?): 
file:///home/novalagung/Documents/some path  with space/ubuntu1204.box (URI::InvalidURIError)
	from /opt/vagrant/embedded/lib/ruby/2.0.0/uri/common.rb:211:in `parse'
	from /opt/vagrant/embedded/lib/ruby/2.0.0/uri/common.rb:747:in `parse'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/action/builtin/box_add.rb:448:in `metadata_url?'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/action/builtin/box_add.rb:76:in `block in call'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/action/builtin/box_add.rb:74:in `map'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/action/builtin/box_add.rb:74:in `call'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/action/warden.rb:34:in `call'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/action/builder.rb:116:in `call'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/action/runner.rb:69:in `block in run'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/util/busy.rb:19:in `busy'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/action/runner.rb:69:in `run'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/plugins/commands/box/command/add.rb:77:in `execute'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/plugins/commands/box/command/root.rb:61:in `execute'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/cli.rb:42:in `execute'
	from /opt/vagrant/embedded/gems/gems/vagrant-1.5.1/lib/vagrant/environment.rb:248:in `cli'
	from /opt/vagrant/bin/../embedded/gems/gems/vagrant-1.5.1/bin/vagrant:158:in `<main>'
```

so, I add some code to handler this error