Simplified commands and improved docs

Got rid of :CurPkg :GoCurInstall :GoCurTest commands and made :RelPkg
:GoInstall :GoTest :GoTestVerbose commands take an optional argument
which defaults to the current package.

Reformatted the Vim help doc for easier visual scanning and added
doc for missing commands.
Merge pull request #31 from riobard/master

Simplified commands and improved docs