Build System Refactoring, Part 2

- improved build scripts for libjasper, proj.4, gdal, geos, postgis and
plv8 by making them more modular (eg. redownloading sources on every
build is no longer necessary)

- added proper dependencies between the different targets

- added postgis and plv8 as targets to PostgresApp

- moved the build variables (library versions) from target to project
for easier maintenance

- bumped version numbers for libedit, libxml2, geos, v8

- updated READ ME file