Added optional support for trailing commas

This also fixes cases where the iterative parser should have produced
kParseErrorValueInvalid rather than kParseErrorUnspecifiedSyntaxError
when expecting a value (after a colon in an object, after a comma in an
array, and at the start of an array.)

This adds kParseTrailingCommasFlag to allow a trailing comma at the
end of maps and arrays. This is part of issue #36, adding optional
support for relaxed JSON syntax.

do potentially precision-losing conversions explicitly

Add PrettyWriter format option for writing array in single line.

This change adds PrettyWriter::SetFormatOptions with a corresponding
bitfield enum PrettyFormatOptions. This allows options affecting the
format of the PrettyWriter to be set. The first option to be provided
is kFormatSingleLineArray, which instructs the PrettyWriter to write
arrays on a single line, rather than breaking them up onto a line
per element.

[simplewriter.cpp] show generated output

This previously affects Writer:: SetMaxDecimalPlaces()

This makes it painfully obvious that writer expects subsequent key/value pairs.

Removed commented code and added an explanatory comment instead

Fix clang example compilation

This reverts commit 6047e3ce.

Implemented feature as in #560 (ints/doubles as strings)

Fix #566 #568