Improve configuration and defaults for document location

[rstoyanchev]

Currently, "classpath:graphql" is the default location for server schema files and for ".graphql" document files for GraphQlClient and GraphQlTester, which leads to issues like #308 where "graphql" under src/test (for documents to test the server with) overshadows the same location with the schema files under src/main.

We need to refine this a bit as follows:

• The server remains with the current default "graphql" on the main classpath
• The Tester can default to "graphql-test" (under src/test) for document files to test the server with
• The Client can default to "graphql-documents" (under src/main), and we can expose an overloaded method on GraphQlClient.Builder to make it easy to have a different location for each remote server.