Trying to solve the Xcode 4.3 problem without breaking Xcode 4.2

Inspired by this pull request that worked in Xcode 4.3 but breaks
previous versions of Xcode.

  https://github.com/square/WaxSim/pull/5

Using the $(DEVELOPER_DIR) environment variable, we can let Xcode figure
out the right root for us to set up the runtime search paths.
use the sdk that is handed in
Use the latest MacOSX
Updated Xcode project to the latest version.
Changed the "deploy target" so it builds correctly for 10.7 in Xcode 4.4

If this isn't set to 10.7, then the resulting binary segfaults.
Set default install locations, so xcodebuild install puts it in a sensible location
Added readme explaining installation, because I can never remember
Merge pull request #1 from sgleadow/install-path-and-install-instructions

Install path and install instructions
Merge remote-tracking branch 'JonathenPenn/master'