Update transport.go to allow usage of http proxies

[sigio]

By setting http.Transport.Proxy = http.ProxyFromEnvironment, irmago will respect system proxy settings.
This allows use in secured or firewalled environments requiring a proxy server for outgoing HTTP(S) traffic.

This code compiles, and seems to use a proxy when setting HTTPS_PROXY

May 14 21:49:29 t480 tinyproxy[331565]: Request (file descriptor 12): CONNECT schemes.yivi.app:443 HTTP/1.1
May 14 21:49:29 t480 tinyproxy[331565]: No upstream proxy for schemes.yivi.app
May 14 21:49:29 t480 tinyproxy[331565]: opensock: opening connection to schemes.yivi.app:443

./irma.bin scheme download
No irma_configuration path specified, using /home/sigio/.local/share/irma/irma_configuration
[21:48:53.597714]  INFO downloading default schemes (may take a while)
[21:48:53.829394]  INFO checking for updates scheme=irma-demo type=issuer
[21:48:54.055701]  INFO scheme is outdated, updating scheme=irma-demo type=issuer
[21:49:32.966370]  INFO checking for updates scheme=pbdf type=issuer
[21:49:33.208570]  INFO scheme is outdated, updating scheme=pbdf type=issuer
[21:49:49.904658]  INFO Finished downloading schemes

Fixes #423

[kamphuisem]

We need to verify that this change will also work with other parts of the IRMA server, because the transport is used for several parts of the IRMA server.
For example, I don't know if proxies will handle SSE correctly with default configurations.
This could also be a breaking change for existing installations, where proxies are configured on a system/user level, but are now not being used by our server.