Move ExcelCompiler functionality to Spreadsheet object while keeping gen_graph working with arguments

[bradbase]

Python 3.4... gotta make that work.

[danielsjf]

@bradbase This fix looks good if it fixes your issues. The Python 3.4 issues seems more of a glitch in the tests than a fundamental error. I've had it too in the past and it disappeared by itself. I'm not really sure what the origin is.

The move to the new ast is nice. The location of that code always bothered me :-)

Regarding the tests, there are a few things that disappear:

• The creation of a spreadsheet from scratch but I don't care. This was mainly to add a new way of testing but it hasn't been used anywhere yet.
• The spaces in sheet names. This was a bugfix and it might be good to check if we can restore this behaviour
• A few renamings that I did, mainly in the spreadsheet file. val -> value; addr -> address. These are small fixes I suppose.

So if I understand it well, with this PR, there is no need to roll back the previous fixes?

[danielsjf]

@Brad-eki, @bradbase, if we can clear out the last remaining points we can merge this one. Since it is quite a big PR, it might be a shame to loose track due to future other PRs that modify similar parts. For instance #236 also interacts with astnodes might also need to be integrated in this one.

[bradbase]

This approach can't work.