Add sign API

[yoh1496]

Implemented sign API

I implemented SignResource class to sign contents with private key which cell has.

The sign API has below features.

• The endpoint is __sign.
• The endpoint is executed with token having p:sign privilege.
• Response with Content-Type application/jose and contains a text formated JWS Compact Serialization .
• The JWS text can be verified with a key which can be fetch from __certs .

Other changes

• Refactoring classes around private key of cell.
• Add some test classes for sign API.
• Add p:sign privilege.

Could you review this?