Wrap sax in trycatch

Resolves #112 by wrapping calls to sax's write and end methods in a try/catch. If we catch an exception, we trigger the exception on the sax parser -- from where it would have originated had the parser been catching parse errors.

This is not complete. The error-handling is currently suboptimal, as it will trigger an error and then never end parsing without user intervention (see L#58 in the test). So, I'm considering making `this.push(null)` automatic.

@kof @alltom Any thoughts?
@fb55 Thanks for your thoughts. I wouldn't recommend strict XML parsing for feeds, but I think namespace awareness is essential. See https://github.com/danmactough/node-feedparser/blob/master/test/namespaces.js Unlike HTML, because of namespaces you cannot have an XML parser that infers any meaning from the names elements if you don't account for namespaces. Just one of the many reasons we all hate XML. :)