Support for PkgCreator to not rebuild a package on every run

- Contribution by Nick McSpadden and some additional changes
- Squashed commits below:

PkgCreator: Clarify output when finding a matching package. (+11 squashed commits)
Squashed commits:
[1a93e4c] PkgCreator: pkg_path is based on request's pkgdir, not always the cache dir
[d2133c7] PkgCreator: Define new_package_request in output vars
[d86370c] PkgCreator: Support for 'force_pkg_build' input variable
[d966809] PkgCreator: set 'new_package_request' that autopkg tool can check for reporting
[4adf77f] PkgCreator: Handle case of a missing PackageInfo file
[fe4437a] PkgCreator: minor cleanup
[96426e5] Remove old reference in xarExpand
[e1e3fdc] PkgCreator: do the rest of the existing pkg check only if we found a pkg
[5b4aa30] Fixed formatting issues that made diffs useless
[05cd339] Fixed spaces-tabs
[1389c81] Added idempotency check

Checks name of package, then bundle id + version to see if there's a
match.
Update CHANGELOG.md
Added the script that generates processor documentation
Merge branch 'master' of https://github.com/autopkg/autopkg
Initial commit for CodeSignatureVerifier processor
PlistReader: when searching for a bundle, ignore symlinks without extensions
Update CHANGELOG
DmgCreator: support for format, zlib level, megabytes options
Update CHANGELOG
Fix issue #91: Hyphen in bundleID not allowed by regex in autopkgserver/packager.py.
Update CHANGELOG
processor wiki docs generation now one self-contained script

- moved to Scripts
- script now performs the git clone, commit, diff and optional push
- no manual steps required
Set force_pkg_build when building new AutoPkg release
Catch recipe run failures when building new AutoPkg release
Missed comma in processor Wiki docs push
Release version 0.3.0.
Bumping to v0.3.1 for development.
Merge branch 'master' of https://github.com/autopkg/autopkg into download-verification

# By Timothy Sutton (11) and Per Olofsson (1)
# Via Timothy Sutton
* 'master' of https://github.com/autopkg/autopkg:
  Bumping to v0.3.1 for development.
  Release version 0.3.0.
  Missed comma in processor Wiki docs push
  Catch recipe run failures when building new AutoPkg release
  Set force_pkg_build when building new AutoPkg release
  processor wiki docs generation now one self-contained script
  Update CHANGELOG
  Fix issue #91: Hyphen in bundleID not allowed by regex in autopkgserver/packager.py.
  Update CHANGELOG
  DmgCreator: support for format, zlib level, megabytes options
  Update CHANGELOG
  PlistReader: when searching for a bundle, ignore symlinks without extensions
Should not fail if expected_authority_names is missing
Updated the description for expected_authority_names