Add JreProxySelector to handle global proxy configuration

The JreProxySelector is used it to adjust the repositories
as they are added to the GrapeEngine. It looks at standard
System proeprties like http.proxyHost and http.proxyPort
and in addition consults standard JRE Authenticator features
for credentials (falling back to System properties
http.proxyUser and http.proxyPassword).

Fixes gh-136