feat: added oauth support: resolves #13

[sdewitt-newrelic]

Description

Adds support for authenticating ReST API calls via OAuth.

• Supports password and client_credentials grant types
• By default will use the ${apiUrl}/oauth_token.do URL but the oauthTokenUrl parameter is supported to allow obtaining tokens from other services
• Uses golang.org/x/oauth2 for performing OAuth 2.0 2-legged flow

Note that we are now using gofmt for formatting code automatically. Therefore, this change includes whitespace changes that are not related to this PR but also don't affect the code.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[jbeveland27]

LGTM 🚀

[jbeveland27]

🚢