DOC: document bs4/lxml/html5lib issues

to summarize this mess of deps and check my thoughts here

### if a user insists on using `lxml` (either with or without `bs4`)
* warning about its inability to deal with the modern web
* warning saying that the user should install `html5lib` and `bs4` so that a page will parse even if `lxml` barfs
* test coverage for failing and passing pages (things that would parse "correctly" before will now fail since the parser will be extremely strict) thus only pages validated by the DTD will even try to parse

(to be fair I was really enthusiastic about `lxml` because of how fast it is but now i'm sort of against it)

### what users should really do
* install `bs4`
* install `html5lib`
* happily parse things into `DataFrame`s with a low amount of stress

### anaconda + `lxml` (no `bs4`)
* no problems (modulo the above warnings)

@wesm maybe you could chime in about what (if anything) you did to `libxml2`/`libxslt` i wasn't clear on the details from the mailing list.
### anaconda + `bs4` + `lxml`
* make sure that you're using `bs4==4.2.1`
* make sure that you're using `lxml==3.2.1`
* workout the details of how to do this with `conda` (i did this already, but it was 2 or 3 AM so I'm a little foggy on the details)

### anaconda + `bs4` + `html5lib` (no `lxml`)
* happy parsing of HTML tables

this will be in a gotcha that will be linked to from a warning at the top of the read html section of io.rst