issue 310 - Added a new property, ignoreSilentSwitch. (#403)

* issue 310 - Added a new property, ignoreSilentSwitch. When true, audio will play even when the silent switch on an iOS device is set to silent. When false, the audio will toggle with the silent switch. Sets the AVAudioSession to either playback or ambient.

* Added ignoreSilentSwitch usage to example app and to readme

* Changed ignoreSilentSwitch to accept two string values, ignore and obey.
This accounts for the case where the user does not want to modify the audio session from a particular video instance. The user would not use the ignoreSilentSwitch property at all in that case.
Also, the audio session will only be updated when the video is unpaused, instead of whenever the video component has updated props. This allows for multiple videos to be on the screen, with the most recent video unpaused (aka played) being the video that has control over the audio session.