Compute # of word of paragraph only once
Code cleaning
Removed function `add_kw_tags`

This function is useless now, because method `drop_tree`
is used for removing nodes from DOM. Before were removed
text nodes next to <!-- comment -->, <script>, etc. so
this function had reason.
Empty list is evaluated as `False`
Removed unused variables `dummy_uri`
Added tests for SAX parser for paragraphs

- Fixed parsing: <body> is also paragraph tag
Typos