1. return cfxaddress when call contract method
2. auto add prefix '0x' when privateKeys not start with '0x'
3. support websocket node url
4. report error when connected conflux-rust verison small than v1.1.1
5. add context 'cfx' and 'cfxsdk' to migration context
6. estimate gas and obtain gasPrice when they not set in config instead of use trffle global default value
This change is