set sdb_path
Better support for multi-version; more options

I have reorganized how the isolinux directories are managed.  Now there
is a top-level `isolinux` directory. Inside of this is one directory for
each major version of RHEL to support. These sub-directories **must** be
named after their major version ('6', '7', etc).

The contents of each major version sub-directory is simply the isolinux
template files for that release.  Note that the isolinux.cfg files still
require specific tokens that will get replaced by `mkvm.rb`.

This commit also adds support for specifying a mount point for /dev/sdb.
```
--sdb 25G,/opt/foo
```
This will create a 25GB disk in your VM, and append `SDB=/opt/foo` to
your Kickstart boot line.  It is still up to you to handle this boot
line in your Kickstart script.

I've also added a `--extra` argument which allows arbitrary additional
boot line elements to be added.
`--extra "one=1 two=2"`
whoops. tidy up README
update changelog
Merge pull request #5 from covermymeds/sdm-issue-3

Resolve issue #3 and a whole lot more