some of pandas.io.tests.test_html.TestReadHtml should be conditioned on presence of lxml
Testing on my local armel box (heh -- bringing now my own media box into the game):  it seems to avoid #4473 but whines about

```
======================================================================
ERROR: test_parse_dates_combine (pandas.io.tests.test_html.TestReadHtml)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/io/tests/test_html.py", line 580, in test_parse_dates_combine
    index_col=1)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/io/html.py", line 840, in read_html
    parse_dates, tupleize_cols, thousands, attrs)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/io/html.py", line 698, in _parse
    parser = _parser_dispatch(flav)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/io/html.py", line 657, in _parser_dispatch
    raise ImportError("lxml not found please install it")
ImportError: lxml not found please install it

======================================================================
ERROR: test_parse_dates_list (pandas.io.tests.test_html.TestReadHtml)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/io/tests/test_html.py", line 572, in test_parse_dates_list
    res = read_html(expected, parse_dates=[0], index_col=0)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/io/html.py", line 840, in read_html
    parse_dates, tupleize_cols, thousands, attrs)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/io/html.py", line 698, in _parse
    parser = _parser_dispatch(flav)
  File "/home/yoh/deb/gits/pkg-exppsy/pandas/pandas/io/html.py", line 657, in _parser_dispatch
    raise ImportError("lxml not found please install it")
ImportError: lxml not found please install it
```
