Add a baseURI  to the global Setup + patched the way JSON is passed to the body

The first part of this pull request adds a baseURI parameter to the globalSetup object. The goal is to avoid this kind of use: 
```javascript
var globalSetup = frisby.globalSetup({ // globalSetup is for ALL requests
    request: {
        json: true
    }
});

frisby.create('Login as test-user')
    .post('http://127.0.0.1:8081/login', {})
    .expectStatus(200)
    .toss()

frisby.create('Login as test-user')
    .post('http://127.0.0.1:8081/logout', {})
    .expectStatus(200)
    .toss()

frisby.create('Login as test-user')
    .post('http://127.0.0.1:8081/test', {})
    .expectStatus(200)
    .toss()
```

by using the following technique:
```javascript
var globalSetup = frisby.globalSetup({ // globalSetup is for ALL requests
    request: {
        json: true,
        baseUri: 'http://127.0.0.1:8081'
    }
});

frisby.create('Login as test-user')
    .post('/login', {})
    .expectStatus(200)
    .toss()

frisby.create('Login as test-user')
    .post('/logout', {})
    .expectStatus(200)
    .toss()

frisby.create('Login as test-user')
    .post('/test', {})
    .expectStatus(200)
    .toss()
```

Then the second part of this pull request is related to the following issue #162 and the way JSON data is sent.