allow-overlap, ignore-placement

Tests for #2348

The first (and third) line does not use expressions, the second (and forth) line use them. These lines should be equal, but they are not in my tests (using eabba2594849ff2097599eef7ba4dbdc2ea86f36):

TextSymbolizer:
![text-allow-overlap-expr-500-500-1 0-agg](https://cloud.githubusercontent.com/assets/249012/3880866/7c68d0d8-2188-11e4-853c-14b5c226f7e7.png)

PointSymbolizer:
![point-symbolizer-overlap-placement-expr-500-500-1 0-agg](https://cloud.githubusercontent.com/assets/249012/3880861/73690390-2188-11e4-855b-1de01259e24a.png)