add tests for #20 and #22

Add a new file, tests/test_md_into_bbcode.js, about #20.
Add a new test case into tests/test_url_formatting.js, about #22.
add tests for 4-spaced code (#21)

modified src/showdown.js, in case of 4-spaced code the language is set to "code"
add new tests into tests/test_code.js about the recognition and the correct conversion of this kind of code
fix tests, in order to avoid some failing tests caused by some behaviours of the 'real' library (not created by us)
now getting more informations from the compiler!

When minifying the Charlotte code, the script now asks the Closure Compiler if our code raises warnings or is syntaxically (:P) broken.
The errors are only notified, the code is always generated (unless build.py is called with --no-minify parameter)
generate_showdown_test now escapes correctly

there were some escaping problems in the Showdown tests: they are now (hopefully) fixed.
Add tests for images and (auto)urls, generate showdown tests and test pages

(These tests are non-regression tests for issues #20 and #22. some Showdown tests related too)

autourl_formatting, url_formatting:  # and @ do _not_ generate [project] and [user] tags, add tests about urlencoded urls and components
images: add "local paths" (/path/to/img) tests
md_into_bbcode: add some links with underscores (yaraon, facebook)
fix #20 comment, #22, assign generic language to code blocks if not provided

* add [img] to the tags whose content must not be converted into bbcode (be sure to activate recognize_bbcode option!)
* fix the #project/@user bug: now they're escaped if into urls or autourls
* now code blocks that don't have a "language" (single backtick, 4-spaced code, 3-backticks with no language) are assigned the "code" default language

boilerplate comment: generated the minified code