FIX: Fix butter
Merge pull request #163 from Eric89GXL/fix-but

FIX: Fix butter. LGTM. tests pass, examples run.
FIX: Fix default fs
FIX: Lower dB for more headroom (75 dB stimuli)
FIX: Keypress demo
Merge pull request #160 from Eric89GXL/fs-fix

FIX: Fix default fs