Add Token authentication

[tomasr78]

Add Tokenand Self Generated Token authentication. The endpoint should receive two parameters Token and ApiKey.

https://v2.convertapi.com/convert/docx/to/pdf?Token={Token}&ApiKey={ApiKey}

The Token authorization with different tokens can be used in the application and it should be an easy way to change Token parameters on runtime. That's the main difference between Token and Secret authentication.

Related https://github.com/ConvertAPI/convertapi-dotnet/issues/30

[tomasr78]

Finally, we have four authentication methods:

1. Authentication using static Secret variable

public static string Secret;

2. Authentication using static Token variable

public static string Token;

3. Authentication using Secret constructor

public ConvertApi(string secret)

4. Authentication using Token constructor

public ConvertApi(string token)

[tomasr78]

Create a token authentication demo and add a description in the readme file.

[tomasr78]

This issue is no longer relevant. A new method for authentication can be found at https://github.com/ConvertAPI/convertapi-dotnet/issues/53.