Proxy support

Hi all and thanks for the useful plugin.

I have added support for publishing from behind a proxy.
Proxy settings are taken from the same file used from Gradle itself.

Please note that I had to fix something in tests ( see https://github.com/enr/gradle-bintray-plugin/commit/b0ebe1b9e6d1e205d007739c8d2d18562005688b ) as I was having compilation issues and a "plugin not found" error.

Cheers, Enrico