-	Modified genericTest.properties
	*	Added property path.mujavaOutput, this defines where mutants will be stored

-	Modified MuJavaAPI
	*	Uses StrykerConfig instead of hardcoded values

-	Modified PrivateStryker
	*	Uses StrykerConfig instead of hardcoded values
	*	Constructor PrivateStryker(JMLAnnotatedClass, String) makes a call to StrykerConfig.getInstance(StrykerConfig.DEFAULT_PROPERTIES))
		if StrykerConfig.instanceBuilt() returns false

-	Modified StrykerConfig
	*	Added method getLastBuiltInstance(), this will return the last constructed instance
		if an instance was not built before this method will throw an IllegalStateException
	*	Added method getMutantsDir(), this method returns the value of property path.mujavaOutput
		replacing the only argument with StrykerConfig#getPathSeparator()
	*	Added static final field DEFAULT_PROPERTIES with the value "genericTest.properties"
	*	Added static method instanceBuilt() to ask if an instance has already been built

NOTES:
1) property path.output in genericTest.properties defines the folder where all outputs will be stored, not only mutants
2) package config wasn't pushed in the previous commit (it's committed in this one)