Email actions implemented

Closes #4273 (closed)

Doing actions via replying to email feature implemented. Allowed actions and their usages:

Method          | Description                        | Usage
------------------------------------------------------------------------
close           | Closes the noteable                | /close
open            | Opens the noteable                 | /open
reopen          | Alias of open                      | /reopen
assign          | Assigns noteable to person         | /assign @meinac
unassign        | Clears assignee from noteable      | /unassign
milestone       | Adds milestone to noteable         | /milestone Version-2
clear_milestone | Clears the milestone from noteable | /clear_milestone
label           | Adds labels to noteable            | /label Inprogress, Bug
remove_label    | Removes labels from noretable      | /remove_label Inprogress
clear_labels    | Clears all labels from noteable    | /clear_labels

In action:

Hi, thank you for the report. /assign @meinac can you take a stab on this? /label Inprogress, Bug

Also this feature can be used while commenting directly from webpage like Rails's r? keyword.