Add support for Spark Micro Framework

[cardil]

Spark is nice small library inspired directly by Sinatra. It will be nice to support it as well.

[cardil]

Unfortunately Spark doesn't support setting port from system property. I will create PR to Spark to add external port setting with -Dserver.port=12345. The same property as in Spring Boot.

[cardil]

This feature is blocked by perwendel/spark#478

[cardil]

There is a walk around for this, obviously. One should use port() Spark method to set port from system property, like this:

int sparkPort = Integer.valueOf(
  System.getProperty("spark.port", String.valueOf(spark.SparkInstance.SPARK_DEFAULT_PORT))
);
port(sparkPort);

Then when testing with Gasper:

@ClassRule
public static Gasper gasper = Gasper.builder()
  .usingSystemPropertyForPort("spark.port")
  .build();

This must be done explicitly in Spark application and can't be used by Gasper globally for all Spark applications.