Support for nengo-bio style excitatory and inhibitory connections

[astoeckel]

This PR integrates nengo-gui with the biologically plausible connection types provided by nengo-bio.

In particular, it

• adds support for the "excitatory" connection type on the client side
• reads the nengo-bio provided connection kind
• reads the nengo-bio provided pre-object metadata to remove dead connections

Note: this PR also includes #1010. While technically independent, this PR should be merged in conjunction with #1009 and #1011.

In case you'd like to test this functionality first, I've merged all my recent PRs #1009, #1011, #1012 into the master branch of my fork of nengo gui at https://github.com/astoeckel/nengo-gui

[tcstewar]

This is also a pretty awesome feature to have in. The code looks nice and clean to me, and this is a nice feature to have.

It's now merged. :)