Add the aggregate form of ST_MakeLine

Usage:
```mysql
SELECT ST_MakeLine(ST_Accum(the_geom)) FROM input_table;
```
Clarify test for ST_PointsToLine, cf. ST_MakeLine
Make ST_MakeLine accept MULTIPOINTS

Still need to write unit tests
Add unit tests for (mixed) MULTIPOINT
Need at least two points when using MULTIPOINTS
Improve doc and point counting
Add one more use case
Delete ST_PointsToLine

All the functionality of ST_PointsToLine has been replaced by
ST_MakeLine.
Replace ST_GeomFromText --> ::Geometry

This is a little easier to read