add browser support

Allow user to specify different browsers example gruntfile:

```
grunt.initConfig( {
    protractor: {
        options: { // Global options
            configFile: "test/protractorConf.js", // Default config file
            args: {
                seleniumServerJar: './test/e2e/selenium-server-standalone-2.37.0.jar',
                baseUrl: 'http://localhost',
            }
        },
        chrome: { // Chrome Specific options
            args: {
                browser: 'chrome'
            }
        },
        firefox: { // Firefox Specific options
            args: {
                browser: 'firefox'
            }
        },
        safari: { // Safari Specific options
            args: {
                browser: 'safari'
            }
        },
        ie: { // Internet Explorer Specific options
            args: {
                browser: 'ie'
            }
        }
    }
}
```