Added a simple validation for new app name

Currently there are no validation for app name. For example when user entered like ```Vendor;Product``` or like this ```Vendor§§Produc``` name, Laravel gives syntax error.

I added a simple validaton for this issue. And PR contains a little improvement for Composer's namespace about this issue.
Then Taylor's 3d841a9738779617eaba740f419bb816992df69b commit should also be changed