do not ignore errors, remove unused error return values

Great care has been taken to make sure the exported API remains
unchanged. Also, tests are fixed
document the invalid return code used in ComputeCode
add ProvisionURIWithIssuer, reuse code for ProvisionURI
adapt comments: Damian confirmed that valid codes are always positive
Merge pull request #1 from PuerkitoBio/master

handle errors, add ProvisionURIWithIssuer method