Display warning when required PHP plugin is not available

[pieterpoorthuis]

This plugin requires the following PHP plugins:

• GMP or BCMATH
• OpenSSL
• JSON
• CURL

The plugin should give the user a warning when a required plugin is not available

[pieterpoorthuis]

Currently, when e.g. GMP/BCMATH is not available the plugin crashes when clicking 'install'. A dialog window pops up that says "Error". The ajax call error is GET 505 error on the following URL /admin/index.php?route=extension/extension/payment/install&user_token=9Z9ynDbwRddiUlXNDS3wGMeKYG2mbEEX&extension=bitpay

[ray-moncada]

Can you add a list of required php-plugin in the readme and in the Admin dashboard? This will help alot.