fmt.Errorf() replace errors.New() and fmt.Errorf()
Merge pull request #27 from lidashuang/develop

fmt.Errorf() replace errors.New() and fmt.Sprintf()
Recommend to use aes-128-cfb.
Remove dependency on leakybuf by importing code.
Create cipher stream for different encrypt method.
New encryption method rc4-md5.
Use shadowsocks-py to test compatibility.
Fix Travis-CI test.
Bump version to 1.1.2
Merge branch 'develop', version 1.1.2