Feature: Adding Custom config file provision.

This patch takes care of issue #41
Adds a way to browse custom config file.
I've tried to make minimal changes to existing ui.
This patch only does this for local encfs volumes as of now.Tried for dropbox and drive too but for some unknown reason couldn't connect to any cloud accounts here. Would definitely add once I figure out the issue.
So, we don't really use fragments in the rest of the code - actually there is issue #60 to convert Encdroid to using fragments. I wonder if it would be a problem that half of the code uses fragments for dialog whereas the rest doesn't. Did you test against tablet form factors?