Add confluence server support

[seanson]

Fixes #2

This PR makes a few basic changes to the provider in order to support both systems:

• Switch from partial domain Atlassian.net configuration to full hostname declaration. I considered either this or a boolean flag to toggle Cloud / Server mode but considered this more consistent with the previous config.
• Split URL configuration as I've seen quite a few Confluence server configuration run the public site behind an SSO gateway and expose the API endpoints privately
• Add configuration for schema, site context and private/public URLs
• Updated documentation to reflect new changes

I've run the following tests:

• Tested terraform apply against resources in both Cloud and Server versions
• Tested make testacc against both Cloud and Server versions

[jayhding]

Looks like this repo is not maintained any more :(