Make ST_GeometryN check for GeometryCollections

Before, this worked:
```mysql
SELECT ST_GeometryN('LINESTRING(1 1, 1 6, 2 2, -1 2)', 1);
-- Answer: LINESTRING(1 1, 1 6, 2 2, -1 2)
```

Now the same request gives the following:
```mysql
SELECT ST_GeometryN('LINESTRING(1 1, 1 6, 2 2, -1 2)', 1);
-- Answer: NULL
```

Unit tests added, including a unit test checking for an out of range
index.
Update doc of ST_InteriorRingN
Update error message
Update doc ST_PointN
Update doc ST_StartPoint ST_EndPoint
Identity function on single Geometries
Update .gitignore
Update doc of ST_IsClosed
Update doc for ST_IsRing
Fix small doc errors
Remove unnecessary local variables
Merge pull request #200 from agouge/st-geomn-collection

Update doc to correspond to website; Add unit tests for ST_GeometryN

Here I modify the javadoc of a few classes to correspond to the documentation on the website as of 7633bf4. I also add some unit tests for ST_GeometryN. See #122.