Build System Refactoring, Part 1

I refactored all the targets necessary for building the base postgresql
binaries and libraries, and tried to fix issues related to OpenSSL and
dylib linking.

- I split the build script phases for openssl, libuuid, libedit, and
libxml2 into multiple scripts for downloading, extracting, compiling to
speed up build times. For example, XCode now no longer redownloads
sources on every build.

- I added the postgresql target as an explicit dependency to the
Postgres target, so that postgresql is automatically built when needed.

- libxml2, libuuid, openssl and libedit are now added as a dependency
of postgresql, so they too build automatically when needed.

- I updated some of the build scripts to avoid misleading errors and
warnings in the build log.

- I changed the postgresql build script so it links to our versions of
the libraries (libedit, uuid) instead of system provided ones.

- I changed the build phases for the Postgres target so only the
postgresql binaries are copied into the package (see issue #116), and
only dynamic libraries -- the static versions are now left out to save
space.

- I added a script phase to the Postgres target that fixes dylib paths.
The script is optimized and should be much faster than the
postgres-dylib-linker target. It also uses @loader_path to address
issue #109

- last but not least, I updated build instructions in the README file

This commit does not yet address the extensions plv8 and postgis, I
will refactor them later.