Stopwords

http://trac.xapian.org/ticket/269#no1
- Gathering the list of Arabic Stop-words
- Include the stop-word lists from Snowball Project

There's a space after the backslash on this line, which confuses the `preautoreconf` script which collates a list of the sources to feed to doxygen, and results in make choking on `docs/Makefile`:

```
make[2]: Entering directory '/home/olly/git/xapian-svn/xapian-core/docs'
Makefile:390: *** recipe commences before first target.  Stop.
```

This may be something GNU make 4.0 is fussier about than 3.x, but generally spaces or tabs after a `\` intended to continue the line are asking for trouble, as they may be interpreted as escaping the whitespace instead.