Split up files so that install only runs on postinstall

Currently installation/checks is ran everytime the binary is required. This fixes that. Would be nice if I could have commit access to this repo too.

I also removed the build since it now depends on mozjpeg.

Tests will fail until there is a tag (`v0.1.6`), see https://github.com/1000ch/node-jpeg-recompress-bin/pull/6/files#diff-6d186b954a58d5bb740f73d84fe39073R12.