Updated to piwik/decompress 1.0 and install PEAR through Composer

PEAR is installed through Composer with the latest version of piwik/decompress anyway. I've removed it from the libs folder after comparing the files. `is_a()` is not deprecated anymore, and I couldn't find any usage of the modified methods `raiseError()` and `isError()` in HTML_Quickform.

I had to require pear/pear-core-minimal on top of pear/pear_exception because there is no transitive dependency. I have opened https://github.com/pear/PEAR_Exception/issues/8