Rewrote the scalar dumper with an emphasis on rigor, based on a
careful reading of the YAML 1.2 spec and testing against LibYAML.

Corrections:

Block style chomping accounts for added the extra newline
added by subsequent elements or at the end.
Fixes #238.

Top-level block scalars are indented.
Fixes dumper side of #221.

Folded style should be reliable now. The old dumper was not aware of
the subtle rules for more-indented lines.
More-indented lines no longer cause extra newlines to be added.
(The old dumper also had an off-by-one error that dropped
the last character from a long first word on a line.)
Fixes #215, #222. Removes #217 workaround.

Literal and folded styles can represent arbitrary "printable" strings,
including leading/trailing whitespace.

NB. For the 'construct-string-types' dumper test, the strings are
dumped correctly. The test fails because a loader bug
loses a newline when using the keep "+" chomping indicator.

Additional:

Added test suite covering several former bugs, new edge cases,
and new expected behaviors. All previous applicable tests are included.

Styles are chosen in a well-defined order.
Plain and single are preferred for single lines under the width limit.
Folded is preferred when a line is longer than the width limit
  (if enabled) and does not begin with a space.
Literal is used for all other multi-line "printable" strings.
Double-quoted is only used when a string is unrepresentable otherwise.

Plain style is relaxed to include more characters and strings.
Progress on #183.

As indent increases, line width decreases down to min(lineWidth, 40).
Setting lineWidth to -1 disables wrapping.
Previously there was unexpected behavior past 40 indent:
narrow lineWidth suddenly jumped to 40,
and -1 (no limit) suddenly enforced a limit of 40.

Updated test for #217.
It now checks that folded style preserves JS functions and regexps.

Replaced String.prototype.repeat with a helper function,
for compatibility with node 0.10.

Rewrite scalar dumper for correctness

Fixes #253: loader should not add an extra newline at the beginning
of block scalars that have an explicit indentation indicator.

Cause: "Detected indent" was assumed logically equivalent to
"did read non-empty line". This no longer holds with an
indentation indicator: indent is detected before any content is read.

Fixes #276: loader should not remove a trailing newline
for block scalars with "+" chomping.

Cause: "+" chomping didn't account for the last content newline
before the trailing empty lines.

Taken from scalar_double_quoted_simple_multiline.yaml

Fix block scalar leading/trailing newlines