fixed CSS inclusion if using shortcode with arguments

Hi,

I am using the the ```[event-list]``` shortcode in my posts and I noticed two little bugs in the current version of your plugin: 

1. The CSS is not properly loaded if you use the shortcode with some arguments. (e.g., ```[event-list show_filterbar=true link_to_event=false]``` )
I fixed this by altering your if-guard in ```Event_List::print_styles``` from ```strstr($post->post_content, '[event-list]')``` to ```strstr($post->post_content, '[event-list')```.

2. If I use the shortcode in my theme's template via ```do_shortcode( '[event-list show_filterbar=true link_to_event=false]' );```, point 1 above is not triggered in either case, because the shortcode is not in the post's content.
For me , enqueuing  the CSS style in ```Event_List::shortcode_event_list``` fixed the problem.

Actually, my solution for point 2 solves also point 1, so I combined the solutions, as you can see in the diff.

Keep up the good work!