Skip tests broken for specific or all platforms

This pull requests contains changes to skip over several brittle tests brought in with pull request #66. Several of these tests cause the Travis CI jobs to fail on Python 3 and PyPy, as seen in the [job following the accepted pull request](https://travis-ci.org/jamescasbon/PyVCF/builds/19434079). These changes will restore the Travis CI jobs to passing status, but with the acknowledgment that certain tests were skipped for certain platforms (Python 3 or PyPy).

This pull request also changes the reporting of several tests aborted by early return statements from the specious status of "ok" to the more accurate status of "skipped".

As the fragility of these tests gets addressed, the skipped status can be removed. For right now, it will help developers to have those tests skipped rather than force them to always expect failing tests and have to parse through the output to see if it includes one of their new tests or not.

This pull request also introduces a new dependency for Python 2.6, the [unittest2 backport](https://pypi.python.org/pypi/unittest2), for the `@unittest.skip` and `@unittest.skipUnless` decorators.