U: add method to download file
U: add 'prepare' & 'ready' methods to control handlers' execution order
U: add 'sandbox' method (uncompleted)
U: add usage
U: add sample code
U: add waffle.io badge
U: complete 'setup' method
U: delete needless codes
fix bugs:

1. runHandler > args => arguments
2. data > catch exception when pass a string parameter
U: fix a little mistake...

update repo's url
Merge branch 'develop'