Caching, environments, refactor, batching

Major changes

• A Slack team set up for googleAuthR package support, sign up via https://goo.gl/forms/d541yrJoDFMrrSJp1
• Default demo project scopes now NULL, set your own Google Project via options() or by setting up environment arguments (#74):
  • GAR_CLIENTID
  • GAR_CLIENT_SECRET
  • GAR_WEB_CLIENTID
  • GAR_WEB_CLIENT_SECRET
  • GAR_SCOPE
• Add ability to return the email of the service account on a GCE instance, gar_gce_auth_email
• Add ability to cache API calls into memory or writing to local disk (#69 and #68)
• Make optional a trailing slash on URIs (#73)
• Improve header checks to be more RFC compliant (#78)
• Use httrs RETRY function for retries that handles handles better (#67)
• RStudio Addin now offers menu to prepopulate scopes for APIs from discovery API.
• Add googleAuthR.batch_endpoint option for specific library batches.
• Remove TRAVIS support in environment arguments
• Add gar_check_existing_token() to improve user feedback on why a reauthentication occurs

Bug fixes

• Fix http2 framing layer error that sometimes happens in misconfigured http2 servers (#87)
• Update minimum dependency to R 3.3.0 to support Shiny function bugs.