Add additional way to exclude code parts using annotations

[GoogleCodeExporter]

This issue will implement a new mechanism to exclude specific classes or (this 
is new!) specific methods or parts of methods from being checked with 
ForbiddenAPIs.

The idea here is to use an annotation, that must have at least 
RetentionPolicy.CLASS or RetentionPolicy.RUNTIME. ForbiddenAPIs will not 
provide such an annotation, it can simply configured to look for any annotation 
type(s), configured as a list of Class parameters.

This allows to run forbidden-apis with ant like:

<forbiddenapis supressAnnotations="org.foo.bar.SuppressForbidden" .../>

(same in maven as a list of class properties).

You have to define the custom annotation (or multiples) as part of your own 
project, so we get no external dependencies. You can also misuse another 
annotation (LOL). The annotation must just be of RetentionPolicy.CLASS or 
RetentionPolicy.RUNTIME. It does not need to be documented. Just SOURCE-only 
annotations won't work (because no longer visible in class files).

You can then annotate classes with this annotation (which does the same like 
excluding the class file using the excludes="...." Ant/Maven config settings) 
or go down to methods or code lines. In fact depending on the type of your 
annotation, you can use it like SuppressWarnings. So annotate any method, code 
line, or parameter [not yet decided if this makes sense] you want t exclude 
from checking (or in Java 8 also types).

Forbidden-APIs will add an "example/default annotation" in its own JAR file, 
but you are not required to use it (especially if you don't want to have a 
compile-time dependency to forbiddenapis).

Original issue reported on code.google.com by uwe.h.schindler on 14 Sep 2014 at 5:08

[uschindler]

I added support for a default (shipped-with) annotation in afce079

[uschindler]

This should work now for Ant, Maven, CLI. Documentation was also updated.