add/move distilled documents
Replace BeautifulSoup with lxml to support Xpath, which is easier to identify the target.
Additional improvements:
1. support + and - operation of targets
2. auto adapt to encoding according to charset field
3. support reflow, which is especially useful for some Chinese sites preformatted by <br> or <pre>
Merge geturl.py into epub.py. Now epub.py supports to download urls, compose epub and test single page