Add iOS and Android basic DRM support (#1445)

This PR adds support for DRM streams on iOS (Fairplay) and Android (Playready, Widevine, Clearkey)

I am neither Android nor iOS developer, so feel free to provide feedback to improve this PR.

**Test stream for ANDROID:**
```
testStream = {
        uri: 'http://profficialsite.origin.mediaservices.windows.net/c51358ea-9a5e-4322-8951-897d640fdfd7/tearsofsteel_4k.ism/manifest(format=mpd-time-csf)',
        type: 'mpd',
        drm: {
            type: DRMType.PLAYREADY,
            licenseServer: 'http://test.playready.microsoft.com/service/rightsmanager.asmx?cfg=(persist:false,sl:150)'
        }
    };
```

or 
```
{
    uri: 'https://media.axprod.net/TestVectors/v7-MultiDRM-SingleKey/Manifest_1080p.mpd',
    drm: {
        type: 'widevine', //or DRMType.WIDEVINE
        licenseServer: 'https://drm-widevine-licensing.axtest.net/AcquireLicense',
        headers: {
            'X-AxDRM-Message': 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ2ZXJzaW9uIjoxLCJjb21fa2V5X2lkIjoiYjMzNjRlYjUtNTFmNi00YWUzLThjOTgtMzNjZWQ1ZTMxYzc4IiwibWVzc2FnZSI6eyJ0eXBlIjoiZW50aXRsZW1lbnRfbWVzc2FnZSIsImZpcnN0X3BsYXlfZXhwaXJhdGlvbiI6NjAsInBsYXlyZWFkeSI6eyJyZWFsX3RpbWVfZXhwaXJhdGlvbiI6dHJ1ZX0sImtleXMiOlt7ImlkIjoiOWViNDA1MGQtZTQ0Yi00ODAyLTkzMmUtMjdkNzUwODNlMjY2IiwiZW5jcnlwdGVkX2tleSI6ImxLM09qSExZVzI0Y3Iya3RSNzRmbnc9PSJ9XX19.FAbIiPxX8BHi9RwfzD7Yn-wugU19ghrkBFKsaCPrZmU'
        },
    }
}
```

**Test stream for iOS:**
Sorry but I can not provide free streams to test. If anyone can provide test streams, or found some we can use, please let me know to also test them.

It has been tested with a private provider and they work, at least with the `getLicense` override method. (An example implementation is provided in the README)

README.md

@@ -415,6 +415,11 @@ Determines whether video audio should override background music/audio in Android
 
 Platforms: Android Exoplayer
 
+### DRM
+To setup DRM please follow [this guide](./DRM.md)
+
+Platforms: Android Exoplayer, iOS
+
 #### filter
 Add video filter
 * **FilterType.NONE (default)** - No Filter
@@ -799,6 +804,17 @@ Note: Using this feature adding an entry for NSAppleMusicUsageDescription to you
 
 Platforms: iOS
 
+##### Explicit mimetype for the stream
+
+Provide a member `type` with value (`mpd`/`m3u8`/`ism`) inside the source object.
+Sometimes is needed when URL extension does not match with the mimetype that you are expecting, as seen on the next example. (Extension is .ism -smooth streaming- but file served is on format mpd -mpeg dash-)
+
+Example:
+```
+source={{ uri: 'http://host-serving-a-type-different-than-the-extension.ism/manifest(format=mpd-time-csf)',
+type: 'mpd' }}
+```
+
 ###### Other protocols
 
 The following other types are supported on some platforms, but aren't fully documented yet: