Improve STL representation detection

Previously, the STL loader relied on a string check at the start of the
buffer to determine if the STL file was of ASCII representation.  This led to
certain false-positives for binary STL files with the magic string present in
their header.  This patch addresses the issue by adding a proper binary check
to the representation detection.
Merge pull request #79 from jdduke/stl_binary_check

Improve STL representation detection