Adding OAuth token support

[chrisforrette]

I tried to use GetCurrentUser from #199 and was getting 401s and realized the Authorization header was being hard-coded and needs to be formatted differently for OAuth tokens, per the documentation here:

https://v2.developer.pagerduty.com/docs/oauth-2-functionality-client-secret#section-using-an-access-token

This PR:

• Adds an authType field to the Client struct, whose value can be apiToken (default) or oauthToken
• Updates NewClient method to set authType to apiToken
• Adds NewOAuthClient method that is identical NewClient but sets authType to oauthToken
• Adds a switch statement in the do method to set the Authorization header appropriately based on the Client authType value.

[stmcallister]

This is great! Thanks for adding this!