ENH: allow to_html and to_latex to take a path for their first argument
not so sure that `to_string` should take this argument; its for a pure string rep, while `to_csv` has all the options for outputing to a file (eg. specialized nan rep, separators, etc)

@y-p ?
 Yeah that's right. More things to maintain for u guys and probably would be confusing for new users. Will take out 
 Agree with jeff, doesn't belong in `to_string`.

I would opt for `isinstance(basestring)` here, 'better to ask forgiveness" not withstanding.
`to_html(FooObject)` should not try to `open(FooObject)`, it should give a sane error message.
 @y-p indeed, thanks.
