Fixed issue with passing options to jit-grunt

Hi @jgallen23

This is fix for #82, but it can be a breaking change because it changes the way of passing static mappings to ```jit-grunt```.

previously:
```javascript
jitGrunt: {
    // static mappings
}
```
after:
```javascript
jitGrunt: {
    // options
    staticMappings: {
        // static mappings
    }
}
```

Also I made all needed changes to ```README.md```