Support named fragments

[stubailo]

Some questions:

1. Do we inline the fragments, or try to keep the original names?
2. What do we do about variables?
3. Do the fragments return their own data, or does the UI component get it passed in from the parent?

More to think about later. The main question is, how critical is named fragment support for the alpha launch? It's one of the flagship features of the Relay API.

[stubailo]

This issue is about using fragments for improved query readability. For including fragments from other places, refer to #148. This feature could be implemented first.

[stubailo]

After talking to @helfer, my strategy is going to be to inline all of the fragments before doing anything else, and fall onto the inline fragment feature.

[helfer]

#251 will solve this completely, afaict.