text: Title: Better support for Pandas.Series
  Body: We should create a LuxSeries object to take on the sliced version of the LuxDataframe, following [guidelines](https://pandas.pydata.org/pandas-docs/stable/development/extending.html#override-constructor-properties) for subclassing DataFrames. We need to pass the `_metadata` from LuxDataFrame to LuxSeries so that it is preserved across operations (and therefore doesn't need to be recomputed), related to #65. Currently, this code is commented since LuxSeries is causing issues compared to the original Pd.Series. 
  
  ```python
  class LuxDataFrame(pd.DataFrame):
          ....
  	@property
  	def _constructor(self):
  		return LuxDataFrame
  
  	@property
  	def _constructor_sliced(self):
  		def f(*args, **kwargs):
  			# adapted from https://github.com/pandas-dev/pandas/issues/13208#issuecomment-326556232
  			return LuxSeries(*args, **kwargs).__finalize__(self, method='inherit')
  		return f
  ```
  
  ```python
  class LuxSeries(pd.Series):
  	# _metadata =  ['name','_intent','data_type_lookup','data_type',
  	# 			 'data_model_lookup','data_model','unique_values','cardinality',
  	# 			'min_max','plot_config', '_current_vis','_widget', '_recommendation']
  	def __init__(self,*args, **kw):
  		super(LuxSeries, self).__init__(*args, **kw)
  	@property
  	def _constructor(self):
  		return LuxSeries
  
  	@property
  	def _constructor_expanddim(self):
  		from lux.core.frame import LuxDataFrame
  		# def f(*args, **kwargs):
  		# 	# adapted from https://github.com/pandas-dev/pandas/issues/13208#issuecomment-326556232
  		# 	return LuxDataFrame(*args, **kwargs).__finalize__(self, method='inherit')
  		# return f
  		return LuxDataFrame
  ```
  In particular the original `name` property of the Lux Series is lost when we implement LuxSeries, see `test_pandas.py:test_df_to_series` for more details.
  Example: 
  ```python
  df = pd.read_csv("lux/data/car.csv")
  df._repr_html_()
  series = df["Weight"]
  series.name # returns None (BUG!)
  series.cardinality # preserved
  ```
  We should also add a __repr__ to print out the basic histogram for Series objects.