Added support for parameterized queries and mutations.

[dblock]

On top of #19, added support for parameterized queries and mutations.

Closes #1.

[ashkan18]

Had one comment/suggestion about how to define variables in our DSL.

On a separate note originally I thought it would be really useful to support parametrized queries and mutations but the more I think about it, it may not be as useful in our case. Basically why would someone do:

client.query(ids: [42]) do
  query(:$ids => :'[Int]') do
    invoices(ids: :$ids) do
      id
      fee_in_cents
    end
  end
end

as opposed to

client.query do
  query do
    invoices(ids: [42]) do
      id
      fee_in_cents
    end
  end
end

As far as i understand, from Graphql server side, they both are processed the same way and there is no benefit in parametrizing the variables over plain query.

Maybe with parametrized approach its easier to share the DSL block?

Regardless its nice to support them, let me know what do you think about my DSL suggestion.

[ashkan18]

oo interesting approach, I wonder if we can make this slightly more DSLy by doing things like:

client.query(ids: [42]) do
  query(:ids => [:int]) do
   ....
   end
end

This means on our end we need to detect query variables and add $ to them when generating query. What do you think?

[dblock]

Good idea, opened #23 for a future improvement.

[dblock]

Two reasons for doing a parameterized query ala version 1 above:

• It passes variables separately in a different field during POST, so you don't risk injection. This is now supported by this PR.
• It allows you to pre-parse the query avoiding to have you parse it every single time. This is yet something we have to support in Graphlient, Allow pre-parsing queries #22.

[dblock]

I rebased it, it's ready to merge again.