fallback to wp-content/uploads if tmp dir is not writable (e.g. shared host)

Hopefully addresses #8
Use WordPress's native `get_temp_dir()`

Will fallback to WP Internal folders if PHP default is not writable.
use wp_filesystem for better compatability
create wp-content dir
try using heckyesmarkdown to prevent parse errors
remind users about zip.so
Merge pull request #13 from jiehanzheng/patch-1

remind users about zip.so