add a "url" property to JSONValue
Merge pull request #16 from tangplin/master

add first and last in JSONValue, add string to double, int etc.
Updated to remove errors in Xcode Beta 3

Changed array syntax to include type in square brackets.
Cast as NSString to work around Swift bug that currently does not
recognize String as a type in (NSObject, AnyObject).
Merge pull request #15 from kyoh/add_url_property

add a "url" property to JSONValue
Merge pull request #18 from krishnan-srinivasan/master

Updated to remove errors in Xcode Beta 3