fix CLOUDSRV-4044 sel field doesn't seem to work for query

# Ticket
https://jira.appcelerator.org/browse/CLOUDSRV-4044

# Description
Root cause:
A bug in  node-acs-sdk util.js 

input:
var query = {
  "where": {
    "module_name": "test"
  },
  "limit": 25,
  "order": {
    "module_name": 1,
    "module_version": 1
  },
  "sel": {
    "all": [
      "module_name",
      "module_version"
    ]
  }
};

in utils.js, sendRequest method will call querystring.stringify(query) to convert the query options to a query string, but if query options contain hash items, querystring.stringify won't convert them to string format, so in acs server side, we will get the following request:

Started GET "/v1/files/query.json?key=0pOweeD1FZgosZjOvajHGWAuStVVc8rI" for 192.168.50.1 at 2014-09-17 13:19:47 +0000
  Processing by FilesController#query as JSON
  Parameters: {"where"=>"{\"module_name\":1}", "limit"=>"25", "skip"=>"0", "order"=>"", "sel"=>"", "suppress_response_codes"=>"true", "key"=>"0pOweeD1FZgosZjOvajHGWAuStVVc8rI", "version"=>"v1"}

and will get the error: 
Invalid JSON in sel parameter: A JSON text must at least contain two octets!

solution:
before pass parameters to api server, sdk will iterate the parameters, if the value of a parameter is a hash, then stringify it.

# How to test
run mocha test at https://github.com/appcelerator/acs-tests/pull/165.

prerequest:
1. download this pull request 
2. setup local acs-node sdk by npm:
npm install ./acs-node-sdk/


Shouldn't be `for(var i in keys) {`?