Build: Enable jscs linelength check, fix many too-long lines

Mostly adds linebreaks to long strings, sometimes renames variables
or edits module and test names to make them shorter. Updates grunt-jscs
to allow disabling for the code from external/jsdiff.js. Keeps excluding
test/deepEqual.js from grunt file, still has 1000+ errors even without
line length check.

Fixes #654