Cleanup warnings and mispractices in slime.el.

* slime.el: `lexical-binding' must appear on first line. Now
attempts to work only from emacs 23 onwards. Adjust
indentation. Remove trailing whitespace. Fix all but one byte-compilation
warning. All function-calls calls to cl.el are now correctly
prefixed with "cl-". Require 'cl-lib.
(slime-edit-definition): No longer use `called-interactively-p'
for known problems.
(hyperspec): non-erroring `require' instead of `find-library'.
(slime-next-line/not-add-newlines): Use `forward-line' instead of
`next-line'.
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* README: Convert to markdown. Use Travis CI badge.