detect and warn if find die() function calls

- ref: https://github.com/GigaOM/gigaom-codesniffer/issues/23
this isn't really the core issue.  Die is not the WordPress way to do things.  From the docs:

>Kill WordPress execution and display HTML message with error message.

>A call to this function complements the `die()` PHP function. The difference is that HTML will be displayed to the user. It is recommended to use this function only when the execution should not continue any further.