BitReader cleanup

[ulmus-scott]

This is a change to make ByteReader not depend on FFmpeg and some minor tweaks to BitReader. The only functional change is in the last commit in regards to how BitReader responds when the data it is reading has been modified since it was constructed (which should never happen). See the individual commits for details.

Checklist

• [x] contribution does not duplicate one of our existing pull requests
• [x] contribution is in a branch rebased against master
• [x] code compiles successfully without errors
• [x] code follows the MythTV Coding Standards
• [x] documentation added/updated/removed where necessary
• [x] commits are logically organised and have good commit messages