Build with --opt-level=3.
Merge pull request #18 from eddyb/opt-level-3

Build with --opt-level=3.
Added a new test to test PNG loading performance and two large test PNGs.
Merge pull request #17 from eddyb/test-perf

Added a new test to test PNG loading performance and two large test PNGs.
Merge remote-tracking branch 'upstream/master'
Replaced link(name=... with crate_id=...
Replaced 'self with a generic lifetime 's
Replaced raw vector and buffer functions with as_ptr and as_mut_ptr