remove PEAR_ErrorStack from PEAR

Hi, I'd like to separate PEAR_ErrorStack from the PEAR package. Since it was alpha in the last (and only) release, this would be fine from a BC perspective. I don't see any chance of PEAR users using it (those who started to use it have all switched to some other error-handling method), and it can be offered in a separate package, for those outside of PEAR that have been using it. As it currently stands, the buzz on the street is all negative, regardless of its merits. Also, I just can't release the package often enough if it is part of the PEAR package. Greg