Per-block access controls for creating and editing block instances. Defaults
behave the same as before, and the framework has final say as before.
See relevant discussion at http://moodle.org/mod/forum/discuss.php?d=36444
Other minor changes: Converted "continue" to "break" inside switch statements
(more to the point, although equivalent), change erroneous (but harmless)
"return false" to "break" on failed addition of block instance, removed some
inline comments from block_base (they were duplicated in PHPdoc)