README: change heading to "Go Ethereum"

README: add config to genesis.json

We have decided to bump the requirement to Go 1.7 because it enables
subtests and allows dropping backwards-compatibility code. This is in
line with Go's support policy. Go 1.6 and earlier no longer receive
security updates.

Changed http:// to https:// on one link in README.md

Changed http:// to https:// on some links in README.md

"fine graned" -> "fine-grained"

(cherry picked from commit e271fd57132763f39c6993a35e6c063aac3af4f1)

README: fix typos

Turns out we actually don't need it, USE_NUM_NONE works
because we also set USE_FIELD_INV_BUILTIN.