Builder: is_writable() and mkdir_recursive()

- Changed writeable to writable, per
http://www.php.net/manual/en/function.is-writeable.php (is_writeable()
is an alias of is_writable() to compensate for common typo/spelling)
- Deprecated mkdir_recurisve function, though, since it is private, it
can probably be removed.
- Changed use of 0777 to DIR_WRITE_MODE and check to ensure the constant
is defined (load the constants config file if it is not)