Add API to re-use request builders

Fixes #182, and is substantial progress toward #185.

The basic idea is that after these changes you can now call ``load()`` on requests builders at any point:

```java
requestBuilder = Glide.with(fragment)
    .from(String.class)
    // apply options here
    ;

requestBuilder.load(url).into(view1)
requestBuilder.load(url2).into(view2)
```