Make PKGBUILD packaging standards compliant.

* replace pointless patch with PYTHON=python2
* simplify git describe command
* move pkgver() function lower, so build chroots can make use of it
* align packages in optdepends using spaces
* use double quotation marks only where it's needed
* unify usage of brackets sorrounding variables
* shorten pkgdesc (kudos to Fedora)
* update README