konoha: Update URL, fix build on 10.8

Is anybody using this at all?
Had to disable mecab as it is no longer in OS X 10.8.
konoha: --HEAD and adding mecab dep for 10.8+

because 10.7 is the last SDK that has mecab.h.
PythonInstalled: set PYTHONPATH, not prepend

An issue could arise when brewing a formula that
has Python 2.x and 3.x support and 2.x is the system Python
but 3.x is a brewed one. The idea about prepending was that
user configured PYTHONPATH could be used in formulae.
Now, instead and if needed, inside the `python do` block
one can still append to PYTHONPATH.