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.
Yes i tested it on my Nexus 7 as well.. and the behaviour is similar to N4, no issues.
I started off using the dialog only. But using normal dialog for browse functionality was troublesome during state changes like orientation change.The browse dialog disappeared on orientation change giving me leaked window exceptions everytime the orientation got changed. Using fragment dialog solved this as it has a special property, setRetainInstance(true), which takes care of most of such issues during state change like orientation change.I ended up using this after a little bit of research over internet.
Maybe this is the first step in dialog to dialogfragment migration.
Secondly.. my review changes for other comments are almost ready. What would do you suggest, shall i submit pull request on master branch or a seperate review branch?