Use $.prop() to get "multiple" attribute

The string 
opts.element.attr("multiple") 
returns:
1) string "multiple" - if element have attribute multiple (<select name="..." multiple></select>)
2) undefined - if element have not attribute multiple (<select name="..."></select>)
It is written in the documentation:
"As of jQuery 1.6, the .attr() method returns undefined for attributes that have not been set."
"To retrieve and change DOM properties, use the .prop() method."
(http://api.jquery.com/attr/)

I am propose use "prop" function. The string
opts.element.prop("multiple")
returns:
1) boolean "true" - if element have attribute multiple
2) boolean "false" - if element have not attribute multiple

After, the "multiple" variable use in check of the condition:
select2 = multiple ? new MultiSelect2() : new SingleSelect2();
Better use true/false variable value than "multiple"/undefined in this condition.

Tested in Opera 12 and IE 8.0.7601
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Duplicating pull request #974 - which seemed to only fix it for 'single' version. Have copied the same condition into the one for 'multi' case. There may be a better way, but in my testing this seems to have worked.
Update select2.css

Removing select2-drop-mask background and transparency to help out corporate-modified IE 8 and IE 7 browsers that disable the "filter" property in CSS.
Galician translation
Update select2_locale_fr.js

Replace accented characters by HTML entities to avoid character set issues.
Merge pull request #1224 from unho/galician-translation

Galician translation
Merge pull request #1225 from arhakal/patch-1

Update select2_locale_fr.js
work around chrome and mutation observer bug. fixes #1099
Added Latvian translation
Merge pull request #1226 from Haran/latvian

Added Latvian translation
Changing to from &apos; to &#39; because &apos; isn't supported in IE8
Merge pull request #1236 from leighhalliday/master

Changing from &apos; to &#39; because &apos; isn't html standard and doesn't work in IE8
Keyboard navigation in choices (tags)
Update select2_locale_lt.js
Merge pull request #1240 from norkunas/master

formatSelectionTooBig fix
Less aggresive keyboard deletes and carret stays
fixed Russian translation
Merge pull request #1243 from boccob/master

Fixed Russian translation
Render formatNoMatches when all ajax results are selected #381
Update select2_locale_zh-TW.js