log statement

This pull request adds the support of two new features:
- first, a new {log} statement that allows to trace scope variables from a template. The {log} statement automatically outputs a new log whenever the variable to trace is changing.
- second, a log() library (in 'hsp/rt/log') that allows to log error/warning/info messages in the same log output as the {log} statement (default is the  browser console)
The log output can also be configured with 'logger' functions that act as 'filters', and that allows to catch logs at application level (very useful for tests, editors or playgrounds..) - cf. new samples provided in the PR

The {log} statement usage looks as follows:
```
# template test(cities)
  {log scope}
  // or with more info:
  {log "top-level scope:", scope}
  <ul>
    {foreach idx,city in cities}
      {log "item #"+idx+":", city}
      <li>{idx+1}. {city.name}</li>
    {/foreach}
  </ul>
# /template
```