'defaults' section in package yaml files
Linux implementation of 'hit build-postprocess --relative-rpath'
new tool: hit build-postprocess --check-relocateable
new tool: hit build-postprocess --remove-pkgconfig

Eventually we should make pkgconfig relocateable, but for now, dodge the issue
and remove the files.
hit build-postprocess --relative-sh-script=...
Fix bug in 'hit build-postprocess --relative-sh-script'
Allow referring to non-existing stages in "before" and "after" ordering properties
Fix display of 'hit purge' command to run in error message
Fixes and upgrades to 'hit build-postprocess'

- Let the check for absolute paths to ignore *.pyc and *.pyo (for now)

- Rewrite absolute symlinks pointing inside artifact, and prohibit
  absolute symlinks pointing outside it
Fixup: Updates to testcase for package.py generating `hit build-postprocess` command
Test and fixup for `hit build-postprocess --check-relocateable`
Correct broken english: relocateable -> relocatable
More rename relocateable->relocatable
Merge branch 'master' into relative_rpath.

The "ignore non-existent build sages" feature has not been merged, so
we are back to where every build stage referenced must be explicitly
defined somewhere.

Conflicts:
	hashdist/cli/build_tools_cli.py
	hashdist/core/build_tools.py
	hashdist/spec/package.py
	hashdist/spec/profile.py
	hashdist/spec/tests/test_package.py
better error reporting for undefined by referred-to stage
Allow build-postprocess handlers to delete files
better error message if PATCHELF is not set
introduce a post_process: stage section
Add Darwin relative rpath fix tool
Merge pull request #226 from hashdist/relative_rpath

Support for building relocateable packages