Bug fixes

I've tested in OSX (10.9.4) and found some bugs.

#### `echo -n` cannot be used in Bourne shell 
Use "printf" instead.
http://stackoverflow.com/questions/17865587/echo-n-works-fine-when-executing-script-with-bash-but-not-with-sh
#### Change "No" as the default choice
When "y/N" is prompted, the capital letter should be the default choice.
http://unix.stackexchange.com/questions/15918/whats-the-standard-used-by-yum-prompt-is-this-ok-y-n
#### Use exit instead of return.
I think it's a serious problem. When I put "n" when prompted, the subsequent commands will be executed as below. (I put `echo "HELLO"` to avoid the git commands from being executed.)
![unnamed](https://cloud.githubusercontent.com/assets/5748806/4515493/728cd23c-4bc0-11e4-951e-cce90525fec2.png)