Adding workspace support

[Spikeophant]

Added a couple functions to manage the workspaces, create, set, and show. Output of show I don't know that I'm happy with:

python3 test.py
(0, 'shared\n', '')
terraform workspace show
shared
But that can be changed.

[eschwartz]

You may want to add some basic test coverage for this.
See this test for tf.apply() as a reference: https://github.com/beelit94/python-terraform/blob/develop/test/test_terraform.py#L178

[Spikeophant]

I would love to get some tests around this, will take a look today.

[Spikeophant]

Added tests, they are passing, wrote a quick .py and ran in an existing tf dir I have for some non production machines, worked as expected. ==================== 30 passed, 9 warnings in 52.13 seconds ====================