Implement basic features with structured layer

[hexoul]

Motivation:

• Initial commit to make Central Dogma client in Python

Modifications:

`Dogma` ---------- `ProjectService` ----------- `BaseClient`
              |                           |
              ---- `RepositoryService` ----
              |                           |
              ---- `ContentService` -------

• Implemented synchronous features are as follows:
  • ProjectService
  • List projects
  • Create a project
  • Remove a project
  • Unremove a project
  • Purge a project
  • RepositoryService
  • List repositories
  • Create a repository
  • Remove a repository
  • Unremove a repository
  • Purge a repository
  • Normalize a repository revision
  • ContentService
  • List files
  • Get files
  • Get a file

Result:

• Above features are available.
• Examples.
• README.

[ikhoon]

The basic structure looks good to me.

[minwoox]

@okue Do you have a chance to look at this PR? If you don't have time, then I think we can just merge this and implement the next feature. 😄

[minwoox]

🎉 🎉 🎉 Thanks a lot @hexoul! Let's go on to the next stage. 😄