Add the mp_client.search(search_hash) method to the collection

[cavi21]

Inspired in @leanucci pull request, I minimize the changes, only add the method in the Collection and in the Client, also add the minitest.

Sorry for this double pull request on the same topic, not sure how is the way to add this to the other one.

Thanks

[cavi21]

I just add the documentation of the search also in the README file.

Just to letting you know
Thanks

[rafaelivan]

Sorry for taking so long to merge these changes.

As the test credentials were not working anymore, I had to change the test account. So, I made a new commit manually including your changes. I also refactored a few things.

[cavi21]

@rafaelivan hey no problem at all! great that you merge this in your repo. Also I contact MP here in Argentina and they deliver a ruby wrapper as a lib a few months ago: https://github.com/mercadopago/sdk-ruby I asked them why not collaborate in your gem, and had no response, just to let you know.