Split `boot.js` into 2 separate files

[!] Actually, `boot.js` does 2 kinds of things
1. Expose jasmine API to global env
2. Add reporters and register spec execution callback to window.onload event

This commit simply splits those 2 things to 2 separate files,
then consider first part as a part of jasmine core, the other as bootstrap.

[-] Invoke execute() method directly if document body streaming is closed