Add feature to support quoted field names in YAML writer.

[baconmania]

Fixes #68.

/cc @cowtowncoder

[cowtowncoder]

Looks good, and although adding features in patches is usually no-no, I think I'll make exception here because 2.9 will probably be the last 2.x version (and conversely 3.0 will take quite a while to be released).

Just one process thing: unless I've asked for and received it, we need CLA before the first merged contribution. It's one time, one page document from:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

and usual way is to print, fill & sign, scan, email to info at fasterxml dot com.
Once it is received I can merge this PR as well as any other contributions.
Apologies for the hassle.

Thank you again for reporting this & contribution the fix!

[cowtowncoder]

@baconmania hiya! I would really like to merge this, but would need CLA if you get a chance.

[cowtowncoder]

Implemented in slightly different, as per #68.