Add blockdiag/seqdiag/actdiag/nwdiag support for RST

This PullRequest add blockdiag-series support for RST.

* blockdiag http://blockdiag.com/en/
* seqdiag http://blockdiag.com/en/seqdiag/
* actdiag http://blockdiag.com/en/actdiag/
* nwdiag http://blockdiag.com/en/nwdiag/

This is optional.
If (block|seq|act|nw)diag is not installed, then disable directives.