closes#172
- added type to ghost.settings()
- added /api/settings?type=<filter>
- added availableThemes to settingsCache
- removed cachedSettingsRequestHandler
- removed /api/themes (including front end)
- changed activePlugins to type "plugin" in default-settings.json
closes#488 and #107
- added dropdown for theme selection on general page
- added GET /api/v0.1/themes to retrieve available themes
- modified settings model to get available themes
- modified updateSettignsCache to remove path from settings.activeTheme