Add flexibility and make it more generic (to do and wish list)

[ger-benjamin]

Ideas todo:

• 11 implement a cancel method see: https://github.com/camptocamp/ngeo/blob/master/src/print/Service.js#L127

More complicated / wanted?

• 1 Somethings about the legend? => maybe later
• 2 A class (extends Layer) to display an ol print mask? => see Add a OL layer mask util class #20

Done or ongoing:
#15

• 1 Remove useless option pdfA => it's optional and means "Allow transparancy" => add doc
• 2 Remove hardcoded datasources in attributes
• 3 Add dynamic attributes (title, comments, foobar, etc) as options of attributes
• 9 Don't limit print format to 'pdf' | 'jpg' | 'png'. (CreateSpecOptions)
• 4 Add a possibility to set the extent in the BaseCustomizer
• 8 Add a getExtent function (with map center, scale, angle, paperSize, PointPerInch, MeterPerInch)
• 5 Add a timeout and error management on the getDownloadUrl method.

#17

• 10 Add a default WMS encoding