Reinstate StaticMatrix * AbstractVector -> StaticVector

[c42f]

This was present prior to the big 0.6 rewrite, and should be reinstated for convenience as the output can be size inferred. This PR is a bit copy-and-paste-ish, though not a lot worse than what's there already, I guess.

This is an associated fix for #131

[coveralls]

Coverage increased (+0.1%) to 69.763% when pulling e7309a5 on StaticMatrix-Vector-matvec into 22b9812 on master.

[c42f]

I think I'll merge this - it's a minor addition which should be pretty harmless.

[andyferris]

Thanks Chris - LGTM