Adding support for generating js bytecode with closure-compiler.

### Usage:
<pre>
Usage: jscompile.py -s src_dir -d dst_dir [-c -o COMPRESSED_FILENAME -j COMPILER_CONFIG]

Options:
  -h, --help            show this help message and exit
  -s SRC_DIR_ARR, --src=SRC_DIR_ARR
                        source directory of js files needed to be compiled, supports mutiple source directory
  -d DST_DIR, --dst=DST_DIR
                        destination directory of js bytecode files to be stored
  -c, --use_closure_compiler
                        Whether to use closure compiler to compress all js files into just a big file
  -o COMPRESSED_FILENAME, --output_compressed_filename=COMPRESSED_FILENAME
                        Only available when '-c' option was True
  -j COMPILER_CONFIG, --compiler_config=COMPILER_CONFIG
                        The configuration for closure compiler by using JSON, please refer to compiler_config_sample.json
</pre>
### Example:
#### Using closure-compiler
<pre>
./jscompile.py -d output_dir -s cocos2dx_root/scripting/javascript/bindings/js -s cocos2dx_root/samples/Javascript/Shared/tests -o game.min.js -j compiler_config_sample.json -c
</pre>
#### Not using closure-comipler
<pre>
./jscompile.py -d output_dir -scocos2dx_root/scripting/javascript/bindings/js -s cocos2dx_root/samples/Javascript/Shared/tests
</pre>