Updated stl_boolean's help text to make it clear that difference mean…

…s minus (subtraction).

New commands:

* stl_ascii - convert binary STL to ASCII STL
* stl_binary - convert ASCII STL to binary STL

* stl_area - calculates and prints the total surface area of all triangles in an STL file
* stl_volume - calculates and prints the total volume enclosed by the triangles in an STL file

* stl_cylinders - generate multiple cylinders using a custom binary input file of line segments that define the central axis of the cylinders.

* stl_bcylinder - print bounding cylinder information about an STL file.

* stl_decimate - simplify an STL mesh file.

* stl_flat - orient the largest surface that can lay flat along the negative Z axis.
* stl_zero - center an STL file at the origin.

incremented $VERSION and updated CHANGES

added CHANGES as a changelog