Clean up WOFF temporary files

[dajiaohuang]

Summary

• Replace the named temp file in BinaryStream::getTempFile(false) with the tmpfile() stream.
• PHP removes the disk-backed temporary file when its stream closes or the script ends, preventing WOFF loads from leaving files in the system temp directory.
• Addresses sys tmp dir left over files #159. The separate custom temp-directory request in Feature request: custom sys_get_temp_dir #154 is unchanged.

Validation

• git diff --check passed.
• PHP CLI is unavailable in this environment; tests and syntax lint were not run.

[bsweeney]

I believe the idea of the original logic was to provide some tracking of temp files created by FontLib. However, without the addition of tracking and removal of created temp files the previous logic was insufficient for a clean run. I think this is an acceptable compromise until we have a chance to better build out the temp file infrastructure.