Added async 'none' option to run all tests in a single casperjs command, and minor updates for casperjs 1.x

Since this plugin executes each test file in a separate casperjs command, some of the command options aren't very useful. For example, the `xunit` option allows all results to be logged to an xml file for integration with CI servers, but this file is overwritten with the results of the last executed test.

This new option `async: 'none'` allows all tests to be executed in a single casperjs command. Using the same example, all test results would be included in the xml file when using the `xunit` option.

This is backwards compatible in two ways:
* By default, tests are run using async series
* The old option format is still supported: `{ async: { parallel: true } }`