Add support for ScriptAliasMatch directives

Much like in PR #483 extend the _scriptalias.rb template so that the
`scriptalias` parameter in `apache::vhost` can contain `aliasmatch`
hashes which get translated to `ScriptAliasMatch` directives. Fixes
issue #480.

Includes basic spec tests and updated documentation in README.md.

Note again that puppet-rspec silently drops `$1` when specifying test
values so we match `FOO` instead of `$1`. From a simple spec test
perspective there shouldn't be a difference and this is better than
having no tests at all.