Perl 5.21.1 and later have a different warning message

Fixes #40

Change-Id: I3308b0e4b46ada8087779493b6e56961b729779d
Test with 5.20 and 5.21 on Travis

Change-Id: I7b28c367e2b8e39eaace5db292c83450f5535c4b
Merge pull request #41 from brummett/5.21-fix

Perl 5.21.1 and later have a different warning message
add primitive copy method to UR::Object
prevent accidental infinite recursion
extract to subtest
allow overrides to be passed to copy
add doc about %overrides
add txtest test helper
use txtest to minimize shared state between subtests
fix incorrect behavior for is_many properties
try to clarify docs
Merge pull request #42 from nnutter/copy

add a primitive copy method to UR::Object