calculate signature properly + fix package.json too be valid json - needed for npm

[forty]

Those commits fix:

1/ The signature is currently calculated improperly

This causes error such as error: 'Invalid API signature'

2/ Currently npm fails to install the module because package.json is invalid:

npm ERR! Failed to parse json
npm ERR! Unexpected token }
npm ERR! File: ../mixpanel-api/package.json
npm ERR! Failed to parse package.json data.
npm ERR! package.json must be actual JSON, not just JavaScript.
npm ERR!
npm ERR! This is not a bug in npm.
npm ERR! Tell the package author to fix their package.json file. JSON.parse

[FRickReich]

still failing.

[forty]

What is failing? Did you pull my repo?