minor fix

* update comments and formatting
use the arrow function for a callback function
refer to constants of the module instead of hard-coding
make the devtools module loader
observe new added messages except updated ones
create a logger for |onMessageAdded|
update the condition for the content filter
fx27: follow changes for the scratchpad editor

* Make Scratchpad use CodeMirror
  see https://bugzilla.mozilla.org/show_bug.cgi?id=912260
fx27: follow changes for the finder module

* Support findbar in e10s
  see https://bugzilla.mozilla.org/show_bug.cgi?id=666816
fx27: add TODO comment

* see https://bugzilla.mozilla.org/show_bug.cgi?id=532150
* see https://bugzilla.mozilla.org/show_bug.cgi?id=891360
fx27: add TODO comment