adds a --branch=<branchname> option. The branchename is passed as BRANCH to the rockspec environment
updated the `--branch` option to work through updating the rockspec `source.branch` field
Additionally a fix for the `git` module, to use command quoting
added help info to build and make commands
Merge pull request #226 from Tieske/branch_option

add --branch=<branchname> option