Warns if plugin class not instantiable

The class returned from NSClassFromString is checked and a warning is produced if it is nil.
Remove JS binding & samples of plugins.
Solve the error during ndk-build when ndk version is r9b & later.
Merge pull request #19 from natural-law/master

Update the plugin protocols code & samples.
Update the sample project for the new version of engine.
Merge pull request #25 from natural-law/develop

Update the sample project for the new version of engine.
Solve the crash in sample project HelloPlugins.
Merge pull request #29 from natural-law/develop

Solve the crash in sample project HelloPlugins.
Solve the memory leak in the JNI methods of protocols.
Merge pull request #31 from natural-law/develop

Solve the memory leak in the JNI methods of protocols.
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