rebase
Merge pull request #14 from heroku/improve-config

All Vault::Config methods have docstrings. All methods have tests
now too.

A new Vault::Config.array method splits environment variables with
comma-separated strings into string arrays.

Vault::Test::EnvironmentHelpers is no longer included in
Vault::TestCase by default. If needed it must be manually
included.

Add additional tests around SSL enable/disable functionality.