Add convenience method Customer#search

Closes #45. Unfortunately the Repo of the original pull-request does no longer exist, so the commits are actually lost and I had to apply them myself, so @edelbalso is not going to be the author of this.
simplify the :from part of the request
Merge pull request #111 from Shopify/add_customer_search

Add convenience method Customer#search
Bump version and update changelog
Merge pull request #112 from Shopify/bump-version

Bump version and update changelog
validate_signature error
Merge pull request #1 from trico/validate_signature_error

Validate signature error