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.
made default tmax 30ms and network default round trip time 20ms to make sure that bid requests aren't rejected if tmax isn't specified in the bidrequest
Merge pull request #109 from rtbkit/latency-exp

made default tmax 30ms and network default round trip time 20ms to make ...
Merge branch 'master' into utf32-experimental