Implement $search parameter

[kbarbounakis]

Description
OData v4 specification describes system query option $search which may be used for searching in entity sets. $search should be implemented also alongside other system query options.
http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-protocol.html#sec_SystemQueryOptionsearch

Solution
@themost/web already implements $search parameter in controllers and services. @themost/express serviceRouter should implement $search by using DataQueryable.search() method.

https://github.com/themost-framework/themost/blob/master/modules/%40themost/data/data-queryable.js#L706