Some subtitle providers require authentication. To configure them,
subliminal accepts the `provider_configs` option to the ProviderPool
class. Add support for it through an `authentication` configuration key.

Having the provider configurations be specified in the `providers` key
would look nicer, but break the case where one wants to authenticate to
specific providers, but still use all the available ones (by leaving out
the `providers` option completely).

Dogpile can spam log messages such as the following at the debug level. Silence it.

2016-07-31 00:44 DEBUG    dogpile.util.readwrite_lock subs            <dogpile.util.readwrite_lock.ReadWriteMutex object at 0x7f693c4c08d0> acquired read lock

BeautifulSoup, as used in some search plugins, returns strings from HTML
document structures in "enriched" form - instances of unicode/str mixed
in with another class. Those strings carry references to the whole
document tree, which can cause issues when attempting to serialize entry
values for saving to the database.

To avoid that, coerce any values whose type is a subtype of text_type,
but not *exactly* text_type, to text_type, with text_type being unicode
in Py2 and str in Py3. That should ensure we throw away any extra
baggage that might be attached but keep the exact same content.

Fix wrong umask calculation in download test

Miscellaneous Subliminal plugin improvements

Convert string-like values to plain strings in Entry