return nil schema on error

[nasermirzaei89]

Golang methods shouldn't return zero value structs on error. So, we should return *Schema on the NewSchema method.

Why:
https://groups.google.com/d/msg/golang-nuts/4yBldwsjZwo/sFcz1KVhU3wJ

[coveralls]

Coverage remained the same at 92.376% when pulling 51495a7 on nasermirzaei89:pointer-return into 02caa89 on graphql-go:master.

[bhoriuchi]

This will break a lot of people's code. So while I agree that it should be done, it should probably be done so in a major release. Additionally you should be checking the error returned before you attempt to access the schema. If the "programmer is to lazy to check the error" then they are probably too lazy to check that the schema returned nil, so they will still get an exception regardless.

[nasermirzaei89]

I agree with merge it for a major release.