Testsuite for XPath expressions

Re-activated luatest and tc_xpath from former times.
Need to check the tests again. Run the test
with `rake test' in the source distribution
Decode html entities

We assume that decoding all html entities (such as
&uuml; does not harm the processing. 

Sometimes we have to parse HTML contents within XML.
Despite the fact that it is more or less always
non-well formed XML, we have some surprising
HTML entities in there. I's probably against all
XML rules to have 'em in there, we accept it.
New option: sp --wd = DIR to change wd
New XPath functions floor() and ceiling()

These come in handy when doing some math.
Small bug fix. It's &quot; not &quote;
New: Mark and sd:pagenumber()

Mark is used in a text block and puts an invisible
mark into the output. After that you can obtain
the page number of that mark with the function
sd:pagenumber().
Minor changes (cleanup and warnings)
Add experimental XPath function merge-pagenumbers

This is not for public use yet, so it's not
documented.
Add XPath "CastableExpr"

To be documented later on. This only handles
"castable as xs:double?" to test whether the
argument is a number.
Two small safety nets for unknown areas.

Its better to do something without assert().
Temporary hot fix.

A project gives me an error about rightskip.prev
being undefined. Need to investigate how this
can happen and how the correct fix will be.