Implements ENV#delete
update spec runner to work with invoke's boolean flags to run specs untranslated
update docs
Untag the specs, remove test and check for null-bytes
Now ENV#delete accepts a block
Merge pull request #524 from davidsch/invoke_untranslated

Update spec runner to work with invoke's boolean flags
Merge pull request #521 from michelboaventura/env_delete

Implements ENV#delete