Add `cytoolz.utils.dev_skip_test` decorator to skip tests for dev versions.

These tests depend on `toolz`, and it is annoying when `toolz` changes in
the middle of a PR.  Now, the things these tests check for only need to be
updated before being uploaded to PyPI.
Move `dev_skip_test` into a module in "tests/" directory.

It was in `cytoolz.utils`, which added `nose` as a dependency.
Add test for `dev_skip_test`
Merge pull request #23 from eriknw/dev_skip_test

Add `cytoolz.utils.dev_skip_test` decorator to skip tests for dev versions