Add bin/buildout --version

Fixes https://bugs.launchpad.net/zc.buildout/+bug/309770
Add support to ignore sections conditionally to a Python expression.

Section titles can now have this form:
 [sectionname:Python expression] # optional comment

If the Python expression evals to False, the section will be ignored.

Expressions have some defaults to support common conditions such as:
 [sectionname: not windows] # ignore this section on windows

Section title lines in the traditional form are still supported of
course:
 [sectionname] # optional comment 
or
 [sectionname] ; optional comment
prep for release.
Merge remote-tracking branch 'buildout/master' into header_expressions
Use more explicit variable names
Added test that verifies #79
Fixed #79

When generating a python interpreter script with an empty `eggs=`,
previously you'd get `sys.path[0:0] = [,]`, which means a syntax
error because of the dangling comma.
Removed buildout.dumppickedversions from the warning.

I *have* retained buildout.dumppickedversions in the doctest with
the qualification 'only part of it'.

Removing the warning *does* make it less obvious for the people
that use buildout.dumppickedversions in the way I thought it
was used most, but apparently the default behaviour of dumppickedversions
was otherwise.
Warn when using an outdated bootstrap that gives you setuptools

Fixes #82

Thanks Marius Gedminas for the fix I could copy/paste :-)

Note that there is no test for this as the test setup doesn't allow for it.
The error that this commit fixes, fixes an error that occurs at
easy_install.py import time, so the fix also has to be on that level, making it
hard to test.
I personally think it's ok like this... ;-)
Added test, but I cannot get it sys.exit to work correctly
Re-instated buildout.dumppickedversions into the warning.
Reworded the warning.
zc.recipe.egg:develop docs fix path -> setup

The required option is actually 'setup' not 'path'.
added meta-recipe support
Added proper exit-code-checking test for restart issue.

When the process is restarted (and results in an error),
we can now properly check the exit code that comes out of
the 'root' buildout process.
Fixes #73: propagate exit code from restart.

A restarted project's exit code is now properly propagated. This makes
buildout more reliable in automatically-run situations.
Simply re-raising SystemExit works, too
Simplification suggested by @mgedmin
Fixing docstring incorrect reference to obsolete command line options
Merge pull request #62 from mgedmin/buildout--version

Add bin/buildout --version
Typo fixes in the changelog