+ small optimization - pass and return const ref
mapnik::value

to_expression_string - add missing specialization
for mapnik::value_integer and remove usage of std::stringstream
+ expression_optimizer (experimental)

at the moment very basic ops are supported e.g

```
expr = 1+1+1 ---> expr = 3
expr = 1+1+[ATTR] ---> 2+[ATTR]

expr = [ATTR] + 1 + 1  ---> ([ATTR] + 1) + 1  ### stays unchaged
expr = [ATTR] + 1/3.14159 + 1 ---> ([ATTR] + 0.31831) + 1
```