style: Enable rustfmt merge_imports and format
This commit enables the rustfmt merge_imports setting
and formats the whole code base accordingly.
Note that the setting is not stable yet, but will be with Rust 1.38.
In the meantime, running fmt on stable will just leave the
changes alone so no problems should occur.
Path handling changed in Rocket 0.4 and this commit updates the way
those paths are handled. It adds an implementation of the
`FromFormValue` trait to the `GraphQLRequest` object. It also changes
the documentation to use the multi-segment query string syntax.
All existing tests pass as expected.
