Change filereader method from readAsBinaryString to readAsArrayBuffer

Original approach by @DrYSG looks better as it uses arrayBuffer natively. This one stringifies the arraybuffer result to minimize impact in lib then converts that to binarystring.

Should resolve ie10/11 attachment issues.
ref: 
https://github.com/daleharvey/pouchdb/issues/900
https://github.com/daleharvey/pouchdb/pull/1078
https://github.com/daleharvey/pouchdb/pull/989