css.js: add context for getSelectors() file read log message
html.js: include context in log message when getHtmlAsString fails
html.js: log context when _processHtmlString fails
html.js: handle links without href attributes

``<a name="">…</a>`` is valid HTML but was causing a type error:

```
[TypeError: Cannot call method 'split' of undefined]
```

This change simply skips such links
ucss.js: ensure doneCallback is defined before it can be called

analyze() could call doneCallback() in several places before it was
certain to be defined
html.js: log the current page being loaded
ucss.js: log fail() at error level
ucss.js: invoke doneCallback in Q.done()

This allows shell invocations to actually exit once the queue has
finished processing without causing library callers to unexpectedly
exit. We probably need to add an errorCallback so the fail() logic
can do the same.
html.js: log initial, final URL for redirects
Disable request pooling

The underlying implementation is unusably buggy — see
https://github.com/mikeal/request/issues/465 — which means that
processing more than a small number of pages (I found somewhere
around 15) will cause ucss to hang with no obvious cause.

Since the actual server response times are almost instantaneous this is
presumably a bug somewhere else (apparently redirect handling); setting
a timeout will cause all 5 (i.e. http.globalAgent.maxSockets) requests
to terminate simultaneously but simply disabling pooling altogether
avoids the problem entirely and makes total execution faster with ~50
total URLs.
Consistency.