Warns if plugin class not instantiable

The class returned from NSClassFromString is checked and a warning is produced if it is nil.
Feature #5027: Add auto bindings
Feature #5027: Add manual bindings
Feature #5027: Add js level scripts
Feature #5027: Add bind script
Feature #5027: Add bind configuration
Feature #5027: Rename bind section to cocos2dx_pluginx
Feature #5027: Update conversion.yaml
Feature #5027: Update genbindings script
Feature #5027: Upgrade manual bindings
Feature #5027: Rebind auto bindings
Feature #5027: Ignore autogenerated userconf.ini
Merge branch 'develop' of https://github.com/cocos2d-x/plugin-x into AddJSBindings
Merge pull request #33 from pandamicro/AddJSBindings

Feature #5027: Add js bindings to pluginx
Feature #5027: Upgrade manual bindings
Merge branch 'develop' into AddJSBindings
Merge pull request #34 from pandamicro/AddJSBindings

Feature #5027: Upgrade manual bindings
add Android.mk for jsbindings
Merge pull request #35 from joshuastray/develop

Feature #5027：add Android.mk for jsbindings
Solve the error config for eclipse in plugin uc.