Adds the Preview API

[floriank]

Adds the Preview API by using Contentful.Preview as a namespace for holding the same structures as Contentful.Delivery

This is also meant for refactoring out the Request code from the modules, as @asaaki suggested in #18.

It's a WIP for the moment, as I have to move a bunch of code. The generic idea is as follows:

import Contentful.Query
alias Contentful.{Delivery, Preview}

# fetches entries from the CDA
Delivery.Entries |> fetch_all 

# fetches entries from the CPA
Preview.Entries |> fetch_all

Edit: Scratch that, it's easier to just follow the idea from @zaeemarshad in #9.

# configure the preview API
# in your config.exs
config :contentful, delivery: [
    endpoint: :preview,
    space_id: "<my_space_id>",
    environment: "<my_environment>",
    access_token: "<my_access_token_cpa>"
]
# then, in your code:
import Contentful.Query
alias Contentful.Delivery, as: Preview
{:ok, entry} = Preview.Entries |> fetch_one("my_entry_id")

This also closes #9 if successful.

[coveralls]

Pull Request Test Coverage Report for Build 859394d66e575354d39b8385f3b608a2c317efd8-PR-31

• 0 of 29 (0.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-14.6%) to 52.985%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/contentful_preview/entries.ex	0	1	0.0%
lib/contentful_preview/preview.ex	0	28	0.0%

Totals
Change from base Build c412f59c3a8d0ef250f5a39bceac3dd3682f8bd3:	-14.6%
Covered Lines:	71
Relevant Lines:	134

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build f13553ecca52a1748ea562665207c0d72df5bbb3-PR-31

• 6 of 9 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 67.568%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/contentful_delivery/delivery.ex	6	9	66.67%

Totals
Change from base Build fab1b3f20e9ac3a770a1d785119d3116851c0165:	-0.7%
Covered Lines:	75
Relevant Lines:	111

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💛 - Coveralls

[asaaki]

Interesting and cheap approach. Feels a bit hacky, but if it works I'm okay with it. Will probably be fine for 99% of the use cases.