Bug 1022725: Rename BingTranslation to BingTranslator and allow overriding URLs. r=florian, a=gavin

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extra : transplant_source : p%AB%FC%86%CB%A61%05pF%C4%98C2I%FDJv-%E0
Bug 1022725: add a mock httpd.js Translation provider for tests. r=florian,felipe, a=gavin.

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extra : transplant_source : %9E%8C%00%21%8D%CC%24%F9O%D5%FE%AEa%9D3%06%E4%10%BE%9C