RESTWS-619 Add SearchConfigTest
* add javadocs to SearchConfig
* add test class for SearchConfig
* move validation of parameters in constructor before setting the instance
properties. so we fail fast if a param is invalid (also fixes a NPE if given a
null supportedOpenmrsVersions).
* simplify SearchConfig.equals()
id, supportedResource, supportedOpenmrsVersion do not have to be tested for
null since they cannot be null (see constructors).
replace obj == null and obj.getClass() != getClass() with instanceof operator
