Fix bug when PERLBREW_HOME contains regex metacharacters

This can readily happen when running the test suite on a Mac, because the
standard TEMPDIR location is a per-user location which can contain "+"
characters.
New feature: "perlbrew install stable"

This installs the most recent available stable Perl, so it is suitable for
recommending to Perl newcomers, even if the recommendation might live a lot
longer than a year.
New feature: "perlbrew install --switch $perl"

Automatically switches to the newly-installed Perl once installation
completes successfully.

This means that a good recommendation for newcomers to Perl who've installed
Perlbrew can be simply

    perlbrew install --switch stable

rather than

    perlbrew install 5.16.2
    perlbrew switch 5.16.2

The latter approach has the additional disadvantage that the appropriate
"latest stable Perl" version changes over time.
Merge remote-tracking branch 'arc/install-switch-stable' into develop

Conflicts:
	lib/App/perlbrew.pm