Fixed typo in README.md
Merge pull request #15 from magnetikonline/patch-1

Fixed typo in README.md
Fixed issue #18 (There was a bug in SpreadsheetService::getSpreadsheetById(), It was passing an xml string to the Spreadsheet constructor instead of a SimpleXMLElement object) and updated README with composer instructions
Updated installations instructions with composer info
Updated README with installation steps
Fixed a failing test (SpreadsheetServiceTest::testGetSpreadsheetById) which was expecting an xml string rather than a SimpleXMLElement object
Added Worksheet::getExportCsvFeedUrl method