AppNexus parsing/handing bid request works; new version of realtime-bidding.proto
parsing ok. (both mobile and single tag BR)
answers: no_bid to requests sent by AppNexus; need to beef up the response -> done
adding creative compatibility check
adding support for providerConfig. support for membership and excluded attributes;
fixed test
Reworked the bid request synthetizer.
Merge branch 'APPNEXUS' of https://github.com/jsulmont/rtbkit into jsulmont-APPNEXUS
Virtual default destructor to avoid compiler warning

The warning was

```
./rtbkit/common/testing/bid_request_synth.cc: In destructor ‘RTBKIT::Synth::NodeObject::~NodeObject()’:
./rtbkit/common/testing/bid_request_synth.cc:267:26: error: deleting object of abstract class type ‘RTBKIT::Synth::Node’ which has non-virtual destructor will cause undefined behaviour [-Werror=delete-non-virtual-dtor]
             delete field.second;
                          ^
cc1plus: all warnings being treated as errors
```
Merge pull request #108 from dohse/virtual-constructor-warning

Virtual default destructor to avoid compiler warning
Fixed the bid request synth build error on gcc 4.6
Added internal support for contexts in the bid request synth.
Added full support for cutoff and generated keys in the bid request synth.
Can now set the generator seed.
Merge commit 'e812914af387cc00da20d609c8b4fe3578c0ef24' into sched-exp