Add Makefile to build the firmware

We need an automated way to build our firmware. For example, it isn't
clear from this repository which version of gluon we're using.
Default to nightly versioning scheme
Update README.md
Pick a commit from master as Gluon 2013.3.1 won't build
Add support for GLUON_BRANCH and build manifest
Fix git rev in default nightly version
Merge pull request #3 from freifunkMUC/build-automation

Build system and git versioning