Add default url parsing to Database() class.

If no url is specified to Database then it will attempt to read a
value from the sunpyrc file.

Also add a url property to the database class to return the url of
the current instance.
revert config after test
no longer copy sunpy.config
load config from file at end of test
monkeypatch FTW @derdon
test excption on no config value being present
seperate the blank config test
Merge pull request #750 from Cadair/database_tweaks

Add default url parsing to Database() class.