Make indentLines return the original Lines object for single-line strings.
Improve util.deepEquivalent and util.assertEquivalent.

Specifically, this commit skips over .comments properties, uses
getFieldValue to properly cope with missing fields that have default
values, and calls JSON.stringify to explain why two objects failed a deep
equivalence assertion.
Strip comment margins instead of just de-indenting their tails.
Better error message for common mistake: lines.getIndentAt(0).
Force one parameter per line if normal printing contained line breaks.
Test trailing comments in function parameter lists.
Rename some comment-tracking properties for clarity in lib/comments.js.
Clean up trailing space after comments.
Add an aspirational test of trailing comment printing.
Upgrade ast-types dependency to version 0.2.18.

This upgrade is necessary for trailing comment tests to pass.
Rewrite comment-adding code to recognize trailing //-style comments.

It's important to note that trailing comments will be pretty-printed as
leading comments, because identifying and preserving all the various
inconsistent trailing comment conventions is pretty much impossible.

Closes #7.
Merge pull request #36 from benjamn/trailing-comments

Better handle trailing comments