New gb flag for building with getgb.io

[mhammonds]

This commit brings support for the gb build system (http://getgb.io/) by passing in a --usegb / -gb flag.

Unfortunately, Gb doesn't currently support arbitrary naming of build output files (See: https://github.com/constabulary/gb/issues/97). This means I couldn't follow the same template used for godep support, and instead I adjusted the documentation to point out that enabling Gb means you must use an absolute path for the --bin flag so gin knows where to run the executable after it is built.