Configuration objects

This introduces the `PushManagerConfiguration` and `ApnsConnectionConfiguration` objects and does away with the `PushManagerFactory` class. `PushManager` instances are now created directly via a public constructor.

This comes in anticipation of having many more configuration options in the near future. In the immediate future, this supports #72 by providing a home for configurable timeout intervals. Later, this will allow callers to configure the connection cycling options [under discussion on the mailing list](https://groups.google.com/d/msg/pushy-apns/HTftu8vH0A0/6jn-xXVFqlUJ).

The problem with the factory pattern was that factories weren't really reusable (they'd be used to construct a single `PushManager`, then discarded) and the pattern demanded an ever-expanding constructor signature to accommodate new options. This approach should be more practical.

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TODO:

- [x] Add unit tests for configuration object copy constructors (4bf02a8)
- [x] Add unit tests for `null` checks in `ApnsConnection`/`PushManager` constructors (8942624)
- [x] More thoroughly migrate docs from `PushManagerFactory` to the connection objects (27ea29c)
> `SSLContextUtil.createDefaultSSLContext(…)`

Why not just `createDefaultSSLContext(…)`?