Merge pull request #8 from Chris00/master

Improve the documentation (cont'd)
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