Fix badly indented let clause.
Add F1 key for help.
Improve indents for some literals. Closes #11.

1. Indent quoted lists and vector literals as:

    '(1 2
      3 4)

    #(1 2
      3 4)

instead of the usual:

    '(1 2
        3 4)

    #(1 2
        3 4)

2. Also, when the customization option racket-mode-rackjure-indent is t,
indent {...} the same way:

    {'key0 0
     'key1 1}

This is intended for use with `#lang rackjure`. It should be harmless
for most Racket code, which rarely uses {}, however it can be disabled
by setting racket-mode-rackjure-indent to nil.

3. Note: This change entailed replacing scheme-mode-indent with our own
racket-mode-indent. And, if someone has indent customizations in their
.emacs, they will need to change them from

    (put foo 'scheme-indent-function)

to

    (put foo 'racket-indent-function)