Cache improvements

[saulojg]

Commit messages should be clear enough. Please reach me if you need further clarifications.

Both changes were properly tested, but outside the test suite since I can't get it to work right now. Here's the test case:

public void testNonCachedReads(){
    Category cached, otherCached, notCached;

    cached = new Category();
    cached.save();

    // read and retrieve from cache
    cached = new Select().from(Category.class).where("id = ?", cached.getId()).executeSingle();
    otherCached = new Select().from(Category.class).where("id = ?", cached.getId()).executeSingle();

    assertTrue(cached==otherCached);

    notCached = new Select().from(Category.class).where("id = ?", cached.getId())
            .setUseCache(false).executeSingle();

    assertTrue(cached!=notCached);
}