liblas add optional laszip

Add optional laszip dependency. Laszip (http://www.laszip.org) is a lossless compression format for las files.

Note that Homebrew is currently building liblas at version 1.7.0, which is [broken](https://github.com/libLAS/libLAS/issues/33#issuecomment-39771404), though a version bump has been requested in libLAS/libLAS#35. `brew install liblas --HEAD` should work ok.