[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>
Whether it is a custom field or not should depend on having the 'custom_' prefix.