Refactor MCP3008 into a callable library(Need to test)

- Add SPI class
- Refactor MCP3008 into a class
Create sensors package
Correct some runtime errors
Add spidev to requirements.txt
Demo the server/client talk ability

- Server has resource /hello which returns a string 'helloworld'
- Client request the resource and get the string

Note: Funny things happen(return None) if the message is a dict and not a pure string. This may require serialize the response on server side before it goes out. Need to research.
Add time resource, and de/serialize on server/client side

- Serialize data on server and deserialize on client, allow transfer of json formatted data
- Add time resource to show system time, in float, unit second, start epoch 1970(for Unix).
Merge pull request #2 from azyuqian/server-client

Server client