refactor examples into its own module

per conversations at https://github.com/awslabs/aws-encryption-sdk-java/pull/6#discussion_r139338345, creating this issue.

```bash
aws-encryption-sdk-java\
-> pom.xml //artifact name for this can be aws-encryption-sdk-java-parent
-> aws-encryption-sdk-java-core
-> pom.xml, src //this generates aws-encryption-sdk-java artifact
-> aws-encryption-sdk-java-examples
-> pom.xml, src //this generates aws-encryption-sdk-java-examples artifact
```

Guys put your feedbacks/proposals in the comments section.

先阅读链接的 pull request 讨论和仓库的 root pom.xml，然后检查现有的 core 和 examples 源代码树。确认所提议的 parent、core 和 examples 模块布局及 artifact 名称；完成的标准是 examples 被隔离在自己的模块中，同时不破坏所述的 build 结构。

由索引模型根据 Issue 内容生成。