add from_pandas and from_dict

[lhoestq]

I added two new methods to the Dataset class:

• from_pandas() to create a dataset from a pandas dataframe
• from_dict() to create a dataset from a dictionary (keys = columns)

It uses the pa.Table.from_pandas and pa.Table.from_pydict funcitons to do so.
It is also possible to specify the features types via features=... if there are ambiguities (null/nan values), otherwise the arrow schema is infered from the data automatically by pyarrow.

One question that I have right now:

• Should we also add a save() method that would write the dataset on the disk ? Right now if we create a Dataset using those two new methods, the data are kept in RAM. Then to reload it we can call the from_file() method.

[thomwolf]

Really great!