Add support for configurable HTTP proxy host/port in PluginManager.java

Formerly by default the PluginMaganer would support no proxy setting.
For servers behind firewall and HTTP proxy this would prevent installation of gitblit plugins.
Add support for specifying the `Proxy-Authorization` header for the PluginManager
Merged #228 "Add support for configurable HTTP proxy host/port in PluginManager.java"