* Accounts in alloc field must already exist

Note that accounts in alloc field must already exist, as pointed out by Simeon Vanov in https://gettoshare.com/2017/10/30/how-to-use-genesis-json-alloc-property/

* Change wording per PR review comment

* README: minor fixups

* Change chainId in genesis block to 8888 from 0 

Change chainId in genesis block to 8888 from 0 per Moriteru in https://ethereum.stackexchange.com/a/28082/40230.

* Replace 8888 with “<arbitrary positive integer>”

Per PR review, replace Moriteru’s arbitrary positive integer 8888 with “<arbitrary positive integer>” in chainId field.

Changed http:// to https:// on JSON-RPC link in README.md

Signed-off-by: YaoZengzeng <yaozengzeng@zju.edu.cn>

* eth: added travis build status for master branch

* README: fix travis badge order, link to CI

* README: add goreportcard.com badge to Readme

* README: fix double github.com

* Remove --fast flag and clarify default

`--fast` is no longer a flag it's `--syncmode "fast"` and that is the default

* Remove --cache flag

--cache=512 is no longer required as of 1.8 as the default has been increased

* README: Minor cache amount fix, mention Rinkeby

missing period at line 119

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.