CMS-2517 GetPartTemplatesRequest and corresponding REST API method

* GetPartTemplatesRequest is using GET now
* getSiteTemplates in PartTemplateListJson was renamed to getPartTemplates
* PartTemplateSummaryJson now extends TemplateSummaryJson
Merge pull request #470 from enonic/CMS-2517

CMS-2517 GetPartTemplatesRequest and corresponding REST API method