Modularize Baigan

[lukasniemeier-zalando]

I am proposing to merge my fork of Baigan configuration.

• the project has been modularized
  • integration with Spring is "just" a module now
  • integration with S3 per module
  • integration with etcd per module
  • added new module to provide Spring Boot AutoConfiguration
  • see individual README files for more information
• simplified configuration formats
  • JSONifyied format (key-value instead of XML-style "alias")
  • introducing namespaces to the configurations, allowing to organize configs
• analyzed typical usage of library and added direct support for certain features
  • chaining sources of configuration (i.e. "fallback")
  • hierarchy of sources of configuration (i.e. namespaces)
  • allowing for sources of configuration to be initialized lazily

Additionally organizational changes have been applied:

• build system via gradle
• change license to MIT

Mind that there is a small set of features which are not yet support by this fork:

• context-dependent configuration values via "conditions"
• serving configuration files from Git repositories

Both may be added.