Set default pattern to src

Fixes https://github.com/alex-seville/blanket/issues/430.

Set the default pattern to `src` if the config is missing or if a pattern has not been explicitly set in the config.

I am looking for guidance on the best way to test this. I have three test cases in mind:

```javascript
describe('default configuration', function() {
  describe('should use src as the default pattern', function() {
    it('missing configuration should still contain a default pattern', function() {
      var blanket = require('../../../src/index')();
      console.log(blanket.options('filter'));
      assert.equal(blanket.options('filter'), 'src');
    });

    it('config missing pattern should still set a default pattern', function() {
      var blanket = require('../../../src/index')({});
      assert.equal(blanket.options('filter'), 'src');
    })

    it('pattern in config should override the default', function() {
      var blanket = require('../../../src/index')({ pattern: 'foo' });
      assert.equal(blanket.options('filter'), 'foo');
    })
  });
});
```

However, these three tests fall down due to the default pattern already specified in the package.json which gets picked up. Any thoughts on a way to get these tests running?

I tried to make this PR against the development branch but it seems like the Gruntfile is incorrectly named and the tests aren't working. Am I missing something? Should I rebase against development and re-compare?