Add new `'` entity to REML

[delphidabbler]

' is a required entity in XML (I think), but isn't supported by REML. May be a good idea to include it.

[delphidabbler]

I think I read somewhere that there's a bug in the HTML / XML implementation in some browser that means it doesn't interpret apos; correctly meaning that the numeric entity is preferred. This requirement doesn't have apply to REML of course.