Optional ZenCoding functions in different modes.

Hello, I want to enable ZenCoding in normal mode only, and find no such option, so I fork the project and do some modifications:

* Separated zencoding installing functions according to their mode

        ( in `zecoding.vim`)
        s:install_plugin --> s:install_plugin_i, s:install_plugin_n, s:install_plugin_v
<br/>
* Added an option to let user control it should be enabled in  which mode:

        let g:user_zen_mode='n'    "only enable normal mode functions.
        let g:user_zen_mode='inv'  "enable all functions, which is equal to
        let g:user_zen_mode='a'    "enable all function in all mode.

And  this is my first try to modify a vim plugin, if these modifications aren't proper, please let me know, thanks.