Require php ext in composer file

Update readme with decimal mode

Fix image source in readme

Testing Docker support on Travis

Improve gmp hamming distance

Trying to speed up hamming distance calculation

Merge pull request jenssegers#3 from TomasVotruba/patch-1

travis: PHP 7.0 nightly added, cleanup

Destroy resource after hashing, fixes jenssegers#2

Allow int as string

Use greyscale for average and difference hashes