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
Bugfix for Resetting Scrollpoint on Multi-select

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.
Keyboard navigation in choices (tags
Less aggresive keyboard deletes and carret stay
Render formatNoMatches when all ajax results are selected #381
Update select2_locale_zh-TW.js
Closes #1228
Merge pull request #1258 from 19Gerhard85/patch-1

Closes #1228
Merge pull request #1255 from demofan/patch-1

Update select2_locale_zh-TW.js
Problem escaped caracter on IE7. fixes #856
#1260 closes #1252 fixes #992 - retrieve option text as value when value is not defined
add margin:0 to select2-cotnainer. fixes #1254
introduce support for separate readonly and disabled modes. closes #1253. fixes #778. closes #1183
Merge pull request #1239 from NielsKuhnel/master

Keyboard navigation in choices (whitespace preserved)
Merge pull request #1062 from bigmihail/patch-1

Use $.prop() to get "multiple" attribute
Merge pull request #1219 from alexhaas/master

select2-drop-mask background and transparency
attr to prop change. #1260
improve offscreen element positioning. fixes #1217. fixes #1218. fixes #1074
Merge pull request #1187 from barryhunter/patch-2

Bugfix for Resetting Scrollpoint on Multi-select