Add implementation to Syndic_error

This ensures that the REPL displays the arguments of the exception
Expected.
Fixes https://github.com/Cumulus/Syndic/issues/11
Use "match" instead of "function"
Factorize out the creation of the XML tree
Be explicit on the use of the helper XML module
Dispatch the author information following the spec

Indeed, the specification mandates that each entry contains an author
unless it contains some sources or the feed contains an author
element.  So the authors are passed around to ensure that each entry
and source has at least one author — or fail.
Extract authors even when <name> is missing

Several feeds use <author>name</author> instead of
<author><name>name</name></author> when solely the author is given.  We
want these to parse.
Fixes https://github.com/Cumulus/Syndic/issues/12
Merge pull request #13 from Chris00/master

Display exception arguments in REPL + factorization and respect RFC 4287 about `<author>`