Various codegen improvements

[thesamovar]

Don't merge yet, for the moment just using this pull request to track the work in this branch.

For a start, resolves #100.

[coveralls]

Coverage remained the same when pulling e3f1854 on codegen_improvement into ed5ec94 on master.

[coveralls]

Coverage remained the same when pulling 6a1546f on codegen_improvement into ed5ec94 on master.

[coveralls]

Coverage remained the same when pulling a1cf422 on codegen_improvement into ed5ec94 on master.

[thesamovar]

So actually there are various things I'd like to do in this branch, including possibly resolving all of issues #104, #99, #96, #80 and #63 - see recently added comments in those issues. I think with all these done we'll have a much nicer templating system with simpler, more understandable Brian code, and more flexible to work with standalone as well as runtime.

[thesamovar]

I think I'm going to put future codegen improvements in the devices2 branch. Now that we plan to merge that branch regularly into master, maybe we should just merge this and delete the branch? @mstimberg want to go ahead and do that?

[mstimberg]

Looks good to me, merging now.