Skipping embedmongo initialization when skipITs environment variable is detected

Instead of the environment variable skipITs, now using a plugin specific parameter "embedmongo.skip" to allow skipping

Skipping embedmongo initialization when skipITs environment variable is set to true, so it play nicer with the failsafe plugin

Closes #36.

Closes #25.
Closes #26.

Random port allocation - randomPort property added to be able to assign random free port at runtime.

Random port allocation - randomPort property added to be able to assign random free port at runtime.

This way we are able to perform concurrent builds on single machine.
Allocated port is set to the system property "embedmongo.port.<maven-artifactId>" where <maven-artifactId> is artifactId of current (tested) maven module.

Slight README improvements