Branch names are stored in filename

[sebastianrath]

Currently branch names are stored as files in ./snow/refs. Since the snow directory is a "system folder" for SnowFS, this doesn't need to be human-readable. Therefore moving the name to the content of the file might be a good candidate to be less restricted by the OSes filename convention.

Git for instance needs git-check-ref-format to ensure the branch name is valid. This shouldn't be a restriction anymore