BTCPay server Upgrade(No facade)

This version supports BTCPay server upgrade regarding removing the concept of facade.
Under the hood, it is removing POS facade and there is only Marchant facade.
and SDK uses POS as default. we need to re-pair with BTCPay

How to upgrade,

1. Upgrade BTCPay server if not yet
2. create a new server-initiated pairing code on BTCPay server (you cannot select facade type any longer)
3. Import the unity package of this release
4. remove the /Resources/pos.txt in the Unity project, which is your old private key
5. set the new pairing code on your GameObject (or BTCPay gameobject if you use the one in unitypackage)

updated WebGL build support

Moved the privatekey inside exe instead of user folder to share common privatekey across users.

first WebGL build support with Bugfix and README updated

Merge pull request #7 from hypergori/webGLsupport

Fix webGL support bug adding dll to link.xml

first WebGL build support

To support WebGL build, we have done the following

1. remove Threading/Task/async-await as much as possible but use Unity CoRoutine
2. remove .net WebSocket class and introduce jslib WebSocket for WebGL and websocket-csharp for others
3. introduce "Json for Unity" package from Asset Store
4. Stripping prevention with link.xml
5. Remove heavy dependency on BouncyCastle and use NBitcoin for Key generation

Still lots of todo, like clean-up (debug logging) and dll for the ease of use by users.

1st version with Coroutine

1.0.0

Merge branch 'master' of bitbucket.org:indiesquare/btcpayunity

beta version 0.01

Importable package for unity