implemented private message's APIs
* GET  /private/timeline
* GET  /private/search
* POST /private/send
* POST /private/destroy
* POST /private/response
* POST /private/remove_response
fixed an issue that the exception was occured when private message was removed
eliminated duplicated functions and added specs.
canl function was added.
can function was moved to util/general.js.
added spec for util/general.js.
unify common logics in util/strings.js.
fixed regression bug caused by last commit
Merge pull request #24 from haushinka2dx/fix-private-and-refactoring

Fixed(implemented) the funciton to handle private messages and refactoring