[ALOY-871] Extend custom formFactor

You can extend custom formFactor dynamically.

check this example,

* xml
```xml
<View formFactor="tall" backgroundColor ="red"></View>
```

* tss
```javascript
".thumb[formFactor=iOS7]":{
	    borderColor : 'red'
}
```

* config.json
```json
{
	"global": {}, 
	"env:development": {}, 
	"env:test": {}, 
	"env:production": {}, 
	"os:ios": {}, 
	"os:android": {}, 
	"dependencies": {},
	"formFactors" : {
		"iOS7":{
			"runtime" : "Alloy.Globals.isIOS7"
		},
		"tall":{
			"runtime" : "Ti.Platform.displayCaps.platformHeight<500"
		}
	}
}
```

* alloy.js
```javascript
Alloy.Globals.isIOS7 = Ti.Platform.name == 'iPhone OS' && parseInt(Ti.Platform.version)>=7;
```

Now you can make any formFactor such as 'SamsunGalaxy3', 'Nexus'.