Fixing the aspect ratio issue

[teunbrand]

This took a bit of refactoring but I believe I may have solved the pernicious aspect ratio - text angle issue as mentioned in #6 .

In brief:

• The ggproto method is no longer responsible for doing the data manipulations. Hence I moved the helpers to a different file.
• Grob creation is now the responsibility of the textpathGrob() constructor, which we can reuse in new geoms we've been ogling.
• The evaluation of the grob, and the measurements/placements, are now delayed until draw time.
• Consequently, we can use absolute units instead of relying on the relative ones, which should give the right angles even under different aspect ratios and even when the device is resized.

Here is a charming picture from a bug I had at some point:

[AllanCameron]

Excellent work Teun. This is the step that will make the whole project viable. I had been working today on trying to get makeContent to work, but couldn't get it to be called when the window was resized unless I had it defined in the global environment. I'll need to study your code to see where I was going wrong.

I'll need to incorporate a few tweaks to fix the letter spacing again: you'll notice that the spacing changes a bit as the curvature varies in the spiral example. The apparent vjust also changes. These are just details though and I'm confident I can fix these quickly.

I'm pretty excited with what we have after this. Nice to see the repo has been starred by some ggplot gurus (hrbrmstr and coolbutuseless).

[AllanCameron]

I'll also need to figure out why that text has flipped in the polar co-ordinates. Again, this looks like it should be a quick fix.

I had a very similar bug on the spiral example to the one you showed. It's quite hypnotic really!