Implemented instance API as an alternative to the static API.
  - Migrated logic to SparkInstace class
  - Replaced logic from Spark & SparkBase to use a default instance of SparkInstance
  - Written instance API versions of all tests
  - Written a test to probe that we are able to run two instances at the same time on different ports
Fixed a typo in InstanceBooks.java (left by mistake some test lines there)
Moved SparkInstance.builder() to Spark.builder()
Moved SparkInstance to Spark.Api
Fixing typos from last refactor
Increased sleep time in ServletTest to ensure that there is time to run tests on Travis
#136 replace all System.out and System.err with slf4j logging
- merged 'jangalinski' slf4j changes
- merged 'ggalmazor' instance api changes
Conflicts:
	src/test/java/spark/servlet/ServletTest.java