At-rules behavior

[616slayer616]

At-rules caus problems as described in these tickets:
google/closure-stylesheets#103 (comment)
gradle-webtools/gradle-minify-plugin#23

In a first step this TODO can be done to prevent an error and just create a warning instead:

closure-stylesheets/src/com/google/common/css/compiler/passes/CreateStandardAtRuleNodes.java

Line 176 in ea59212

reportError("@" + charsetName + " removed", node);

Ideally, the behaviour can be further modified:

• prevent removing of rules (maybe parameterized)
• correct behavior for example given in google/closure-stylesheets#103