Create epub versions of tutorial and ref manual.

Pandoc can create epub verions of the markdown files. Since the docs
are lengthy, epubs are handy to have around. Two rules to create epub
versions of the reference manual and the main tutorial are added here.

Signed-off-by: Noufal Ibrahim <noufal@nibrahim.net.in>
auto merge of #10226 : nibrahim/rust/docepub, r=alexcrichton

Added two new rules to create epubs out of the tutorial and reference manual source files. This is useful and doesn't add any new dependencies to the build process.