remove dependency to missing imghash library

[markus-wa]

This PR replaced the imghash library with hash/fnv.

If I run go mod tidy in my project I get the following error:

$ go mod tidy
github.com/markus-wa/demoinfocs-golang/examples/heatmap imports
    github.com/dustin/go-heatmap tested by
    github.com/dustin/go-heatmap.test imports
    github.com/jteeuwen/imghash: git ls-remote -q https://github.com/jteeuwen/imghash in C:\Users\mwalt\go\pkg\mod\cache\vcs\899a7f2d063a36fd4ddfd213140d24b9bce8d21ec528de64aa73ce33841599f9: exit status 128:
    remote: Repository not found.
    fatal: repository 'https://github.com/jteeuwen/imghash/' not found

imghash has been deleted by the owner (along with all his other repositories) a while ago, see jteeuwen/go-bindata#5

Instead of removing the tests like #5, this PR replaces the library with a custom imgHash() function.