imp: qweb compiler now condenses whitespaces

This is a pretty big breaking change: the html output by qweb is now
different.

The main goal is to optimize the compiled templates:
- condense all consecutive whitespaces into a single spaces
- if possible, drop completely some text nodes, based on the following
  heuristic: if a text node is only composed of whitespaces, and
contains at least one linebreak, then it can be dropped.

This leads in my benchmark test to an improvement of about 10%, in
rendering speed and in memory consuption.

Note: whitespace here means anything that matches the \s regexp: newlines,
tabs, ...

close #8

doc/qweb.md

@@ -25,3 +25,11 @@ result = qweb.render("sometemplate", { name: "world" });
 - t-on directive
 - t-widget, t-props, t-key
 - t-ref
+
+## Note on white spaces
+
+White spaces in a templates are handled in a special way:
+
+- consecutive whitespaces are always condensed to a single whitespace
+- if a whitespace-only text node contains a linebreak, it is ignored
+- the previous rules do not apply if we are in a `<pre>` tag