Add Showdown test generation script

This script is useful to automatically build the tests/test_from_showdown.js file.

The input text is into tests/from_showdown/*.md files, the result into tests/from_showdown/*.bbcode files
update tests from Showdown.

Update the tests taken from the Showdown project: that way we can check if md2bbc still works correctly
add the md files

brother of the previous commit
fix the build files, save the built test pages

build/build.py now gets the list of .js test scripts correctly.
build/generate_showdown_test.py now saves the from_showdown js test script in the correct position
automatically generated the test pages
add readme
Merge branch 'alpha' of http://github.com/alfateam123/md2bbc into alpha