remove unused fields / packages
use GSON as constant to avoid reinitialization
deprecate singleton methods
minor refactoring
introduce SailthruHttpClientConfiguration interface

This interface will provide different options to customize HTTP
specific flags like: connection timeout, socket timeout, address reuse
and tcp nodelay
getSoReuseaddr() as false
add java doc
bump junit version
create http client only after setting sailthruHttpClientConfiguration
use @Test annotation in SailthruUtilTest tests
add tests for DefaultSailthruHttpClientConfiguration and SailthruHttpClientConfiguration
Merge pull request #30 from infynyxx/custom_connection_timeout

9497: provide interface to customize HTTP specific configurations