travis: use cpanm --notest
Add id method to Blob class
Changes: update
Add TreeBuilder API
Only return wrapper objects when in non-void context
TreeBuilder: test get where there's no entry
TreeBuilder: Return undef when using get and the target doesn't exist
TreeBuilder: remove repo from write arguments

We don't need it, since we provide repo to new.
TreeBuilder: Update documentation
TreeBuilder: Fix compiler warnings