Use "defined" in DBM::Deep::Array::PUSH

This way it is possible to push 0's and empty strings.
Iterate over each argument to PUSH, even undef ones

The previous commit would exit the loop upon an undef element, whereas this version always iterates over all of the arguments.
Add test for pushing non-true values; add James Stanley to contributors list
Merge pull request #1 from jes/master

Use "defined" in DBM::Deep::Array::PUSH