Specify lexical-binding on the header line, to keep package.el happy

I noticed that @david-christiansen committed `lexical-binding` declarations in f64d71813155. The style of these declarations stops package.el from parsing info out of the buffer, which is why the description of `idris-mode` on MELPA is currently blank.

So this pull request fixes that, and also fixes up a couple of malformatted description lines.

But separately, I'd just like to confirm that `idris-mode` does not support Emacs 23, right, despite the presence of `idris-compat`?  If so, I'll submit an additional pull-request specifying a package dependency on Emacs 24.

Cheers,

-Steve