Override methods of AmazonSQS, AmazonSNS to provide asynchronous send
messages
use single thread executor instead of cached thread executor
use the same thread executor for Amazon SQS and SNS
redone asycn sending messages
Revert "redone asycn sending messages"

This reverts commit 1bf37bfd02e981d14c52227eb836956a1054fe61.
refactoring
fix tests and implement general nevado interface for AmazonSQS and
AmazonSNS
implement general nevado interface with implementation for AmazonSQS and
AmazonSNS
remove unused methods
change way of checking Amazon client
remove unnecessary sources
make _isAsync as final
return null messageId if it's isAsync
Merge pull request #50 from shlasouski/master

Override methods of AmazonSQS, AmazonSNS to provide real asynchronous send messages.

Nice work Serg!