Extract ac-js2-mode-map as a separate variable for easier customization
Fix broken modification of completion-at-point-functions

Using add-hook was incorrect here: given an initial value of
'(completion-function-1 completion-function-2), the code would
result in 'completion-at-point-functions being set to
'(ac-js2-completion-function t), which was clearly not the intention of
the code.
Don't auto-enable ac-js2-mode

Following elisp conventions, code should not auto-enable
itself: it should be possible for a user to have a large number of
packages installed without all of them being turned on. Indeed, this is
the case with the Emacs standard library.

This change therefore removes the autoloaded `add-hook` code here in
favour of advising the user how to explicitly enable the mode.
Merge pull request #10 from purcell/various-fixes

Various fixes