Adding custom user headers at exec time

[jdesiloniz]

This PR includes code to let users of the api to include custom headers (i.e.: "user-agent", ...) in their API requests. To achieve this, several changes have been introduced:

• Interpreters now perform a natural transformation from our algebras to a Kleisli that carries the header data.
• New exec functions have been implemented to make the access to the API easier for users (i.e.: execK exposes the inner Kleisli, exec allows the same behavior as before while letting users add headers to the requests as an optional parameter, and execFuture allows easier access to Future versions of the API for scala-js users).

Could you please review, @raulraja @juanpedromoreno? Thanks!

[codecov-io]

Current coverage is 86.64% (diff: 100%)

Merging #39 into master will increase coverage by 0.26%

@@             master        #39   diff @@
==========================================
  Files            21         21          
  Lines           301        307     +6   
  Methods         299        305     +6   
  Messages          0          0          
  Branches          2          2          
==========================================
+ Hits            260        266     +6   
  Misses           41         41          
  Partials          0          0          

Powered by Codecov. Last update a714e61...e0ba903

[juanpedromoreno]

LGTM!

Let's wait until @raulraja thoughts around this ;)

[raulraja]

Excellent.