Added alpha argument with default 0 (use optimal alpha value)

This fix will enable pgr_alphashape/pgr_pointsAsPolygon to set arbitrary alpha value.
If specified alpha value equals 0 (default), then optimal alpha value is used.

This fix is related to issue #235, but independent from pull request #236(hole polygon case).
So, the following image's alpha = "0.0001" shows invalid result because of hole polygon case.

![pass_alpha](https://f.cloud.github.com/assets/629923/2301593/6c79890c-a14e-11e3-82c0-e99219148d84.png)