Use registermultitask to enable additional configurations

Currently it's only possible to have a single configuration for running jasmine_node. This makes it difficult to have varying configurations for running in dev to running in continuous integration environments.

This PR enables the jasmine_node section of `gruntfile.js` to contain any number of configurations for running jasmine_node. This is achieved by registering jasmine_node as a ["multi task"](http://gruntjs.com/api/grunt.task#grunt.task.registermultitask). 

### For Example

```js
grunt.initConfig({
  jasmine_node: {
    dev: {
      jUnit: {
        report: false
      }
    },
    continuous: {
      jUnit: {
        report: true,
        savePath : "./build/reports/jasmine/",
      }
    }
  }
});

```

## Sharing Configs

If you have multiple targets, it may be helpful to share common configuration settings between them. This is supported by using the `options` property:

```js
grunt.initConfig({
  jasmine_node: {
    options: {
      forceExit: false,
      specNameMatcher: 'specs',
    },
    dev: {
      jUnit: {
        report: false
      }
    },
    continuous: {
      forceExit: true,
      jUnit: {
        report: true,
        savePath : "./build/reports/jasmine/",
      }
    }
  }
});

```
In this example the `continuous` and `dev` targets will both use the `specNameMatcher` specified in the `options`. The `continuous` target will override the `forceExit` setting to `true`, whereas the `dev` target will inherit the `false` setting from `options`.

## Running tests

If you have a single target, running the tests remains the same: 

`$ grunt jasmine_node`

If you have multiple targets, you can run the desired target by running with the appropriate `:flag`. For example, with two targets, dev and continuous, running the dev target is as simple as:

`$ grunt jasmine_node:dev`

Additionally, running `$ grunt jasmine_node` when multiple targets are configured will run each target in turn.