make install script more robust, hide output

- add checks for curl, wget, mysqladmin, svn, tar
- add descriptive feedback
- changed set -ex to just set -e
cURL is just used to test if the version of wordpress is available, but cURL is not required. It's just an optional feature to check if the version-built-url exists.

e.g. if a user requests version 4.2 right now, it will check if `http://wordpress.org/wordpress-4.2.tar.gz` returns a 200 and error out if not.

If the system doesn't have cURL it skips it.