Added Support for Impersonation Tokens

• Added support to create, list, and revoke impersonation tokens.

Added support for sudo

• Administrator logins can now sudo as a different user (#92).

Added Support for Project Snippets

• Project snippets can now be created, updated, deleted, and listed.

Added Support for Comments and Merge Request Commits

• Added V3 support to RepositoryFileApi createFile() and updateFile() (#65)
• Added support for commit comments (#67)
• Added methods to fetch commits associated with a merge request (#68)