Added support for string based ids

I really love the way scaloid lets you build UIs without using XML. But unfornatetly support for ids is bad. The only way I was able to give UI elements ids in scaloid is by first defining the id in a xml resource. 

In `res/values/ids.xml`
```xml
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <item type="id" name="email" />
</resources>
```

In `MainActivity.scala`

```scala
SEditText().id(R.id.email)
```

Since this seems rather annoying and not on par with the rest of scaloid I introduced support for dynamic ids based on strings

In `MainActivity.scala`:

```scala
SEditText().id("email")

find[SEditText]("email")
```