provide options for #provisioning_uri

options are passed to the ROTP::TOTP constructor, which currently makes
use of :issuer and :interval option keys
Merge pull request #19 from rossta/provide_options_for_provisioning_uri

Provide options for #provisioning_uri
specs for Model#provisioning_uri
Merge pull request #20 from rossta/specs_for_provisioning_uri

specs for Model#provisioning_uri