Move rembed to rembed.core

Make rembed into a namespace package, so that other things (e.g. the
Markdown plugin) can be dropped into it.
Update example in README
Newline
Merge pull request #25 from matt-thomson/issue19

Move rembed to namespace package

Closes #19
Prepare v0.3.0 release
Whitespace
Merge pull request #26 from matt-thomson/v0.3.0-dev

Release v0.3.0