Add new projects to introduce abp module for aelf system contracts interaction.

[EanCuznaivy]

Take an example, developers who are using abp framework to build their own services can simply reference AElf.Client.Token and make their module depends of AElfClientTokenModule, to interact with aelf Token Contract via injected ITokenService.

The total framework has already been established.

• fulfill all system contract services.
• docs.