Support a DIAGRAMS_SANDBOX environment variable.

This can be set to any sandbox directory where diagrams-lib and
diagrams-rasterific (for example) are installed.
Add diagrams-rasterific to travis config.

This pulls in HEAD for diagrams-rasterific from github along with the
other backends.
Merge remote-tracking branch 'acowley/master'

closes #10

Conflicts:
	diagrams-builder.cabal