fix a few compile warnings

1, const qualifiers on return types make no sense and will trigger a compile warning: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]

2, class HistogramImpl has virtual functions and thus should have a virtual destructor

3, with some toolchain, the macro __STDC_FORMAT_MACROS is predefined and thus should be checked before define

Change-Id: I69747a03bfae88671bfbb2637c80d17600159c99
Signed-off-by: liuhuahang <liuhuahang@zerus.co>
Merge pull request #269 from huahang/patch-2

fix a few compile warnings