Combined DecodedConnection and Connection

- The behavior of a connection now depends on pre and post,
  either of which can be a node, ensemble, or neural population.
Combined DecodedConnection and Connection

- The behavior of a connection now depends on pre and post,
  either of which can be a node, ensemble, or neural population.
Fixed bug in rasterplot

- Never should have used an underscored attribute...!
Merge branch 'connections' of https://github.com/ctn-waterloo/nengo into connections