[processing] Add crs selection panel to scripts

CRS selection panel can be added as widget from scripts:
![crs_panel](https://f.cloud.github.com/assets/1368688/1485485/c375e824-4715-11e3-9b4d-bfbc4de3a185.png)
Simple type 'crs' as parametr type:
```python
##outCrs=crs
```

Default CRS is WGS84, but it can be changed by passing optional value which can be any of string definition supported by QgsCoordinateReferenceSystem.:
```python
##outCrs=crs EPSG:2180
##outCrs=crs proj4:+proj=tmerc +lat_0=0 +lon_0=19 +k=0.9993 +x_0=500000 +y_0=-5300000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs
```