Fix issue #63: Add support for runtime evaluation in conditional CSS

This patch bring support for runtime conditional CSS.
```
@if (eval("com.foo.bar()") ){
}
```

The support of runtime conditional is only there for backward compatibility and has some constraints (present in old CssResource):

- cannot define a constant inside a block of a condition evaluated at runtime.
```
@if (eval("com.foo.bar()") {
   @def MY_CONSTANT 15px;
}
```

will throw an exception during the compilation.

- you cannot use `&&`or `||`operator:

```
@if (eval("com.foo.bar()") && eval("com.foo.foo()")) {
  
}
```
won't work. Use the operator in the eval function instead:

```
@if (eval("com.foo.bar() && com.foo.foo()")) {
  
}
```
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