Moving models to a separate directory and standardizing the use of config object

[AvinashBukkittu]

The current NER and SRL processors (including the NER trainer) are not efficiently designed. Particularly, in the initialize method, we create a model and use config object in a non-standard way. That is, we do not follow any pattern on how to access the config object. Also, creating the model inside the initialize makes the processor to be tightly coupled with that model and hence to use a different model, we need to tweak the initialize method. This defeats the purpose of reusable processor. This issue is created to address the following

1. Create a directory of models like for e.g., BiRecurrentConvCRF, LabeledSpanGraphNetwork etc. The processors should import the models defined here based on a value in configs passed to the initialize method.

2. Standardize a way to use config object. Each processor is currently using config object in its own way which will lead lot of repetitiveness and unnecessary complications. To address this issue, we need to ensure a set of standard keys to use for most common configurations like model_path, resource_dir etc. This, however, does not restrict the user to follow this pattern but introduces a principled way to use the config object.