add ".txt" extension for text files in project root directory

This makes them easier to read on a Windows system (though the
files still contain unix newlines which notepad itself can
not handle).
rewrap LICENSE.txt (no text changes)
add enumeration to clauses of license terms
replace <ORGANIZATION> placeholder with "copyright holder" in LICENSE.txt

This matches the text available from OpenSource.org [1] and expresses the
same intent as the original text. Therefore I conclude this does not
represent a "license change" but merely textual clarifications.

[1] http://opensource.org/licenses/BSD-3-Clause
fill in placeholders in LICENSE.txt

All commits are licensed under the 3-clause BSD license but contributors
keep copyright for their own changes thus all contributors are
copyright holders.