Add config options to control caching and downloading.

This commit adds three new config values, all only concerning the loading. These values makes it possible to configure whether servers should be downloaded to calculate md5 checksums, get the size and patch them. Whether all servers should be cached, or only servers that are patched.
Fixes #16
Merge pull request #18 from totokaka/fix-issue-16

Fix issue 16
Merge master