re-use OAI response as a source
Periodically clean-up resource triples
clean update every 5 updates
Select correct MIME type for server sample extraction response

Currently the Server always responds with a "application/xml" MIME type
when trying to extract triples from a Wikipage, no matter which format
is selected in the form.

When it comes to formats which are not TriX, the rendering on modern
browser fails due to incorrect files w.r.t. the declared MIME type of the
HTTP response.

With this pull request, the "application/xml" MIME type is selected only if
TriX format is picked, otherwise "text/plain" is used.
Issue #53: improve external links handling

External links are URLs contained in square brackets.
If the text inside square brackets is not a URL then the string is plain text.

Change is twofold:
- restrict external links to URLs and not URIs
- create text nodes when external links creation fails
Merge pull request #97 from ninniuz/server_extract_mime_type

Select correct MIME type for server sample extraction response
Merge pull request #98 from ninniuz/53_wikiparser_square_brackets

Issue #53: improve external links handling
keep cache of date per priority in case priority items are empty
This is used when we terminate and want to store the latest modification date
Merge remote-tracking branch 'origin/master'