Drop the dependency on the Spring Framework

[jigga]

1. All Spring dependencies have been removed.
2. OkHttp client is now used to interact with Confluence APIs, and the Authorization header is added via an Interceptor.
3. Jackson is the only JSON processing library.
4. The jvm-test-suite plugin has been applied to separate unit & integration tests.
5. The Gradle publish-plugin has been updated to the latest version (1.2.0).
6. Gradle integration tests for the confluence task first publish the current SNAPSHOT version of the plugin to mavenLocal, so it's no longer necessary to do it manually before running integration tests.
7. Using built-in Java utils for reading and writing files.

Test evidence

1. Project's README.md on Confluence

2. Tree structure when generating pages using the fileTree

3. All unit tests pass

4. All integration tests pass

[qwazer]

Thanks for contributing @jigga and @imanushin FYI Looks like gradle team change process of publishing. Now they are review plugins before publishing. I started publishing of v0.10.0

Response from Gradle:

Publishing plugin com.github.qwazer.markdown-confluence version 0.10.0 Thank you. Your new plugin com.github.qwazer.markdown-confluence has been submitted for approval by Gradle engineers. The request should be processed within the next few days, at which point you will be contacted via email.