Changed pointers to references where nullptr is invalid

[mitchellstern]

This standardizes the code base according to @neubig's suggestion in #201. A few minor warnings are also fixed.

[neubig]

Thanks! This looks good. This will break external software toolkits that rely on DyNet's C++ API, so I think we should notify people on the dynet-users mailing list, and then pull in a day or so.

[mitchellstern]

Sounds good, thanks! I think the major API change to note is that all builders (e.g. all RNN and softmax variants) and all trainers now take a Model& instead of a Model* during construction. In addition, a few utility functions (e.g. in dict.h and pretrain.h) were modified to take references instead of pointers where appropriate.