Added a size threshold to size classifier

Only paragraphs of sizes where there are fewer than X instances will be treated as headings. This is to stop the classifier accidentally classifying vast swathes of a document as headings.
Converted size and self.size_cutoff to floats

Otherwise, comparison is string length and doesn't properly trigger.
Added instruction parsing for external and internal links

This eliminates the encoding problems found in some reference links that look like this:

HYPERLINK "http://www.medscape.com/content/2003/00/45/63/456300/456300_tab.html" \l "Table 2." \n tablesTable 2
</ext-link>

Solution is parsing instruction that finds text between \l and \n as text of hyperlink.

Fixes #4
Add NoneType check to avoid null reference in TEI Manipulator
Add test for complex links with escape characters (#4)