Add optional parameter isBase64, as per the Readme

verifyReceipt and verifyAutoRenewReceipt now take a second, optional parameter called isBase64, that describe whether receipt is already base64-formatted. This is consistent with part of the README.
Update README

Remove the ambiguity about the number of parameters of both verifyReceipt and verifyAutoRenewReceipt. 
Fix 'typo'
Merge pull request #6 from Dashlane/master

Adding parameter isBase64