Updated to use API Key + Secret auth, updated whmcs's admin instructions wording

* Changed coinbase_post_json() to use proper HMAC-SHA256 signed API calls
* Added api key/secret param ($api_auth) to callers of coinbase_post_json()
* Updated instructions provided at WHMCS admin panel
Merge pull request #1 from nezzario/master

Updated to use API Key + Secret auth, updated whmcs's admin instructions...