Set SKIP_INSTALL=yes for all embedded library targets.

When Xcode performs build+archive, it actually installs any targets for
which SKIP_INSTALL=no in their defined INSTALL_PATH:

* The static libraries are installed in DESTROOT/usr/local/lib
* The Mac OS X framework triggers an Xcode warning, as it has an
* INSTALL_PATH of @rpath
* plcrashutil is installed in /usr/local/bin
* etc.

This results in these libraries being incorrectly included in the output
of Build + Archive if PLCrashReporter is used as a nested project
dependency.

To resolve the issue, this commit sets SKIP_INSTALL=yes on all library
targets; all are meant to be used as embedded frameworks or static
libraries, and as such, they shouldn't ever be installed in a common
DESTROOT (and in the case of the Mac OS X frameworks, attempting to copy
them to "@rpath" is an error).
Generate a Mac OS X CrashReporter.framework.dSYM, and include it in the release build.

Issue: PLCR-508
Include the current branch's unmodified sources in the release DMG.

Issue: PLCR-508
Add a sanity check for local modifications when performing a release build.

This can be overridden by setting PL_ALLOW_LOCAL_MODS=1 in the
environment prior to executing xcodebuild.

Issue: PLCR-508
Include a top-level "API Documentation.html" symlink in the release build output.

Issue: PLCR-508
Only enable the 'local modification' check for Release builds.

This allows debug 'Disk Image' builds to proceed if there are local modifications.

Issue: PLCR-508
Generate a dSYM file for all build types, not just release builds.

This allows us to include the dSYM when generating release artifacts
using the 'debug' build configuration.

Issue: PLCR-508
Copy the project LICENSE to the generated release output.

Issue: PLCR-508
Generate a .zip archive in addition to the release .dmg

Issue: PLCR-508
Fix calculation of the modification count.

The check in question would always fail, as the returned 'wc -l' result
contained whitespace, breaking the comparison against '0'.

Issue: PLCR-508