Enable parallel execution of Apex test classes

This branch adds support for running Apex tests in parallel.  This functionality is turned on via an environment variable.  To enable parallel test execution, do the following:

1. Set the `TEST_MODE` environment variable to `parallel`
2. If your test class naming convention differs from %_TEST, set the cumulusci.test.namematch to a like query on class name which will match your test classes (i.e. `TEST_%`)
3. If building on your local machine with TEST_MODE=parallel, you will need to install the simple-salesforce python library.  The recommended way is to set up a python virtualenv and then activate it:

    `virtualenv SOMEDIR`
    `source SOMEDIR/bin/activate`
    `pip install simple-salesforce`