Remove Browser Connection driver extension and add Network Connection driver extension

This is related to this issue: https://code.google.com/p/selenium/issues/detail?id=7315

It's basically the Ruby version of this commit: https://github.com/SeleniumHQ/selenium/commit/8629dc6ec6ddd96bbe0975ee86d0c4c2f4b9ebb2

HasBrowserConnection is a driver extension that is only an option for the Android driver. The
Selendroid JSON Wire implementation doesn't support the `browser_connection` method anymore,
so all code related to this driver extension can be removed from the Ruby bindings.

The new Selendroid JSON Wire method `network_connection` returns the connection type on a GET request and changes the connection type on a POST request. Currently you can only switch between Airplane mode and All mode. (The implementation is here: https://github.com/selendroid/selendroid/blob/master/selendroid-standalone/src/main/java/io/selendroid/server/handler/NetworkConnectionHandler.java.)

The new HasNetworkConnection driver extension can handle all types even though that doesn't work yet. In case an unknown type is returned `network_connection_type` returns this unknown type value. I just wanted to have something for this possibility. Can someone think of something better to return if this happens? 

I did have some problems with getting the integration specs running for Selendroid. Could someone help verify that these specs pass?

I haven't signed the CLA yet. I will do that once these changes are approved.


I think it makes sense to put the integer-symbol mapping into a constant hash to avoid duplicating it. You can use `Hash#invert` in one of the methods to get the inverted mapping.

Also, this method should raise an `ArgumentError` if given an invalid argument, instead of passing nil across the wire. Should also have a test for this case.