Added support for SpEL cache keys

Spring Expression Language or SpEL (http://docs.spring.io/spring/docs/current/spring-framework-reference/html/expressions.html) is very useful for defining method keys right in the @Cacheable annotation as described here: http://docs.spring.io/spring/docs/current/spring-framework-reference/html/cache.html
For example:
@Cacheable(value = "default", key = "#p0.concat('-').concat(#p1)")
public String cachedMethod(String param1, String param2) {
..
}

The current code fails asserting that the type of the key is assignable from GaeCacheKey, I added the conversion from a raw string (returned by SpEL) to a new GaeCacheKey