Easier parallel uploads caching

[odeke-em]

This PR addresses issues #315 and #77. It also fixes the bugs and complexity in PR #210.

• Introduces a flag --v for verbosity during push to see step by step information.
• Introduces $GOMAXPROCS to be the number of max parallel uploads. Default is 4.

To test it out:

$ go get -u github.com/odeke-em/drive
$ cd $GOPATH/src/github.com/odeke-em/drive
$ git checkout easier-parallel-uploads-caching && git fetch --all
$ go get ./drive-gen && drive-gen
$ drive push -v [paths...]