Add position information in XML
Add accessor for position
Add information of position in all of parsing
New interface of Syndic_error
Add to_string function
Change RSS 1.0 with new handle of error
Raise not found if data does not exist in tag
Use new interface of error for RSS 2.0
Change Atom with new handle of error
Change handle of error in root of document
Remove useless exception
Aftereffect of last commits
Remove useless polymorphic variant
Use "link list" consistently for "links"

Closes https://github.com/Cumulus/Syndic/issues/37
Merge pull request #35 from Cumulus/error

New handle of error
Merge new error and tree definitions
Adapt the XML generation to the new tree definition