Improve properties definition

Change most properties fieldnames to better fit Matlab conventions (i.e.
fig is now Figure, plot is now Line).
Make size an optional argument

Make width & height optional
arguments, window size is used by default now.
Allow exporting to multiple formats

- Format argument can now be a cell array of format strings, so a
figure can be printed to multiple files in a single call.
- Add ‘eps’ format as shortcut to ‘epsc2’ (PostScript Color Level 2).
- Tweak printing of raster formats to try to improve output quality.
Do not change Axes LineWidth by default

Remove default property prop.Axes.LineWidth = 1.
Add basic usage and licensing to README
Merge pull request #1 from jlcorra/develop

Merge development changes into master