Ignore Gemfile.lock since this is a gem

"When developing a gem [...] Do not check your Gemfile.lock into version
control":
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/
Convert readme to markdown format
Convert to more conventional gem build process
Add missing file removals to teardown
Move OS-specific code to library.  Crontab by cmd.

Extract out all OS-specific code into per-OS library files.  This will
make it easier for folks to add support for a new OS, as well as make it
more clear how well we support each OS.

Per-user crontabs they are now always manipulated via the crontab
command, never by directly manipulating the files in cron's spool
directory.  This eliminates some redundant code, and ensures that cron
is notified of changes on platforms (like Solaris) where cron does not
monitor the spool directory for changes.

Some tests were moved around, some were fixed or improved.