Simplify the Experimentation API

[Vijay2win]

Create a simple API instead of dealing with client object.

• For example experiment can be the first class citizen

experiement = Experiment()
experiement.new_model("resnet")

• Client can be hidden for the user and the server can be configured as an environment variable

• When updating the Model metadata, simplify the API

client.update_metadata({"dataset/path":data_dir, hyperparmas':params})
or 
client.update_metadata(key='hyperparmas', value=params)

• While logging the metrics, we might want to simplify the api

experiment.log(key='loss', value=loss) 
# Automatically increment the step if not explicitly passed like experiment.log(step=1, key='loss', value=loss)

• While of calling for get_metrics() allow users to specify the columns they would like to see
  client.get_metrics(columns="step, loss, accu")
  0, 2.3025460243225098, 0.1328125
  1, 2.3025460243225098, 0.1328125
  2, 2.3025460243225098, 0.1328125

[diptanu]

Some of these changes have landed via #49