Remove Tag and Hole from drawRect when tagLength == 0

I also fixed how the default UIAppearance values were being loaded. That fixes the problem in which I'd have <code>tagLength = 0</code> but the following code would override it to the default:

```
float tagLength = [[AMTagView appearance] tagLength] ? [[AMTagView appearance] tagLength] : kDefaultTagLength;
```

Since I had set <code>[[AMTagView appearance] tagLength]</code> to <code>0</code> somewhere in my own code, the ternary if would load the value of <code>kDefaultTagLength</code> when adding a new tag.