index.js: Added initial version of callback
test.js: Added in callback function

NOTE: there might be a bug in the index decodePayload function, I will continue to look into it
index.js: Only return on error
parser.js: removed assertions to arrays

remove NOTE
index.js: replaced return [err] with callback
parser.js: replaced error return expects
index.js: Added return so as to not continue after encountering an error
parser.js: Added test case and corrected types of
parse.js: Added isLast statements to decPayload tests
Merge pull request #8 from sweetieSong/add/decodePayload-Callback

Add/decode payload callback