Specify PEM file path OR contents

[samharnack]

[larskuhnt]

Could this please get merged, I'd really love to be able to use certificate data instead of a file. The failing specs of this request are failing on master as well.

Thanks

[mklb]

.. and I´m trying to get pushmeup to work with a pem file in an environment variable on heroku Thought I´m doing it wrong.. Would be very nice to merge this !