Merged Fig <-> Nofig, now truely offline

[harshpurwar]

This PR merges the two classes plotlyfig and plotlynofig (previously created for documentation needs). This also now works truly in the offline mode.

There is no need to even signup to chart studio and get an API key if a user wants to use plotly_matlab in the offline mode.

Request to reviewers: Please test this thoroughly before merging it. You may use the following code or write your own complex version:

trace1 = struct(...
  'x', [1, 2, 3, 4], ...
  'y', [0, 2, 3, 5], ...
  'fill', 'tozeroy', ...
  'type', 'scatter');
trace2 = struct(...
  'x', [1, 2, 3, 4], ...
  'y', [3, 5, 1, 7], ...
  'fill', 'tonexty', ...
  'type', 'scatter');
data = {trace1,trace2};
response = plotly(data, struct('filename', 'basic-area', 'fileopt', 'overwrite', 'offline', 1));
url = response.url

[harshpurwar]

Also added an option to suppress writing a file writeFile (defaults to true) in both fig2plotly() and plotly(). This can be used to just get an object of the plotly figure and not write any file to disk (in offline mode) or chart-studio (in online mode).

[gilbertogalvis]

I like much the code developed from @harshpurwar. Great style!