This commit removes the "helpers" package by moving functions from the
package into the other packages that use them.
CheckPayloadSignature() and ExtractParamater() are simply moved to the
"hook" package. I'm not sure of the usefulness of having these
functions exported, but I left them allow for now.
ValuesToMap() is moved to the "main" webhook package and renamed to
valuesToMap().
Tests were moved into the "hook" package since we only test
ExtractParameter() right now.
This commit closesadnanh/webhook#12.
