Update spie-journals.csl for new specification

Authors list is now contracted (et al.) when more than 3.
In-text citation is now a superscript. Additional labels (page, chapter, etc.) are now not possible.
Changed format of list numbers in bibliography, from "[2]" to "2.".
Added "accessed" to corresponding date of webpages.
Merge pull request #950 from myurkin/patch-1

Update spie-journals.csl for new specification