bash and zsh version, and a few clean-up

* Makefile - let's compile *.c and *.hs with only one command (tested on MacOSX, should also work for Linux)
* prime.bash - bash implementation using only bash internal commands.
* prime.zsh - zsh implementation using only zsh internal commands. (sqrt optimization)
* Few clean up:
    - Adds .gitignore
    - Removed a.out
    - add shebang (#!) lines and give x bit for the scripts.

Thanks.
-- Ryoichi