Changes in the layout and in the options.

[vfabregat]

The summary changes are:

• Changed the layout to use flex.
• Changed the styles
• Added the following options:
  • devTools: to show the devTools
  • backgroundColor: Added the option to change the default background color
  • buttonsColor: Option to change the default buttons color
  • textFontSize: Added the option to change the text font size
• Allow to set notification without a duration
• The title is not required anymore

Fix #32