[discuss] SOAP API: add method for searching within issues

Add method 'mc_search_issues' to SOAP API. Filter is in JSON format.
It is also possible to filter by custom fields.

The example of filter with 'os' == "CentOS" and custom field 'abrt_hash' == "hash_content":

{"abrt_hash":"hash_content", "os":"CenstOS"}

Signed-off-by: Matej Habrnal <mhabrnal@redhat.com>
Do we want to support project_id here as a first class parameter and within the filter itself?  In the advanced filter case we may have even multiple projects within the filter.  We can keep this, but should document the behavior.