DOC: Fixing Doxygen groups.

Added these two classes to the STLMeshIO group.
BUG: Now managing Endianness.

The STL format assumes little endianness in the files:

    http://en.wikipedia.org/wiki/STL_(file_format)#Binary_STL

Here we introduce conversions between System endianness, and little
endianness when reading and writing in BINARY format.