0.4rc1: conesearch documentation error

This pull request fixes `conesearch.conesearch()`, which appears to be broken in 0.4rc1:

```
In [1]: from astropy.vo.client import conesearch
In [2]: from astropy import units as u
In [3]: result = conesearch.conesearch((10.7, 41.3), 0.1 * u.degree, catalog_db='Two Micron All Sky Survey (2MASS) 1')
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-7-d50adf279af7> in <module>()
----> 1 result = conesearch.conesearch(c, 0.1 * u.degree, catalog_db=my_catname)

/home/gb/bin/anaconda/lib/python2.7/site-packages/astropy-0.4rc2.dev9406-py2.7-linux-x86_64.egg/astropy/vo/client/conesearch.pyc in conesearch(center, radius, verb, **kwargs)
    189 
    190     return vos_catalog.call_vo_service(conf.conesearch_dbname,
--> 191                                        kwargs=args, **kwargs)
    192 
    193 

/home/gb/bin/anaconda/lib/python2.7/site-packages/astropy-0.4rc2.dev9406-py2.7-linux-x86_64.egg/astropy/vo/client/vos_catalog.pyc in call_vo_service(service_type, catalog_db, pedantic, verbose, cache, kwargs)
    846 
    847     if pedantic is None:  # pragma: no cover
--> 848         pedantic = votable.conf.pedantic
    849 
    850     for name, catalog in catalogs:

NameError: global name 'votable' is not defined
```

This bug was not caught by the unit tests because all of them pass `pedantic` as an explicit argument to `conesearch.conesearch()`, i.e. the line containing the bug was not covered.

While at it, this pull request also fixes three examples in the conesearch documentation which are using the outdated way of resolving coordinates by name, i.e. using `coordinates.ICRS.from_name()` instead of the new-style `coordinates.SkyCoord.from_name()`. (`ICRS.from_name()` appears to have been removed from v0.4 without deprecation warning?)