HTTP and SSL improvements

Updates the WebSocket and REST transport layer with AndroidAsync library.

Implements SSL pinning against simperium.com's SPKI.

Fixes some async bugs around database indexing and reporting of download progress during an initial bucket index download.

Uses Android Gradle's build flavors feature to produce a library compatible with testing that includes all of the mock components. Production flavor is named `client` while testing compatible flavor is named `support`.

When using the library from source code reference the flavor and build type to get testing and debugging support:

    dependencies {
        compile project(path: ":Simperium", configuration:"supportDebug")
    }
`JSON_CONTENT_TYPE` is unused.