Fixed issue #13 and #16

I added a way to disable the plugin with a toolbar button. 
I do also allow the user to use the built in coverage suite. 

There is one issue (non-blocking): 
When idea.system.path is null, how should I built the IDEA cache path? (See row 157 in src/cc/takacs/php_codeverage_display/display/DisplayHandler.java)