v0.12.1

1. update the example of sending transactions to make it more clear.
2. set default address prefix to ckt instead of ckb, and expose address configuration setter in the core.generateAddress method.
3. fix typo in the docs.
4. add private key and lerna tool guide in the docs.
5. use TextEncoder and TextDecoder from the global object if possible.
6. update system cell info test cases according to the testnet.