Create MANIFEST.SKIP file

[xsawyerx]

While creating a MANIFEST file is okay (as long as it is done via the dist builder), having a MANIFEST.SKIP file is better.

It should be done before the create_MANIFEST is issued, so it takes the SKIP into account.

Also, it should check if the file already exists, not to overwrite any other file.

[xsawyerx]

t/boilerplate.t should be included in the create_MANIFEST_SKIP