Extract non-data files in dvExtractFileIds

[leeper]

Supplemental files (codebooks, etc.) are storedin otherMat attributes in the DDI. Here's an example:

<otherMat level="study" URI="http://thedata.harvard.edu/dvn/dv/leeper/FileDownload/Study+1+%28Lab%29+Key.docx?fileId=2228888">
<labl>Study 1 (Lab) Key.docx</labl>
<txt>Study 1 data key</txt>
<notes type="vdc:category">Codebook</notes>
</otherMat>

This will need to parse the URI in order to extract the fileId, since it is not listed as a separate attribute (as in fileDscr nodes).