Add RequestParams#add

This is a shortcut for adding a parameter which can have many values; useful for communicating for something like Rails, where array parameters are non-unique.
DRY
doc
Merge pull request #275 from usepropeller/request_params_add

Add RequestParams#add
Merge branch 'master' of https://github.com/loopj/android-async-http

# By Clay Allsopp
# Via Clay Allsopp (1) and James Smith (1)
* 'master' of https://github.com/loopj/android-async-http:
  doc
  DRY
  Add RequestParams#add