added initial version of xMatch service; still WIP
fixed remote test for XMatch.query
Xmatch.query: return the text content directly, not the HTTP response object
PEP 8
Added trivial non-remote tests for XMatch (error checking).
xmatch: minor change
removed unnecessary kwarg `module` to ConfigurationItem
improved readability and debuggability
xmatch test: outsource expected CSV output to file
renamed XMatch.is_available_table -> XMatch.is_table_available
fixed NameError
more duck-typing
make XMatchClass.get_available_tables return a list of str directly

makes it easier to use and simplifies the API and implementation
make XMatchClass.query return an astropy table
fixed error in xmatch/setup_package.py
cache available tables collected by XMatchClass
added docstring to XMatchClass.query
use astropy unit arcsec in XMatchClass.query
XMatch.query: support Table objects for cat1 and cat2
test for XMatch.query: check the error message as well
XMatch: made the tests more readable

use keyword args in XMatch.query calls