Update READMEs

Updated the main `README` to include information about the Cloud SDK and list the Go components currently in development.

Updated `gcloud_apis/README` to include installation and usage instructions.

Added [GFM](https://help.github.com/articles/github-flavored-markdown/) syntax.
Should probably just say to...
$ export GCLOUD_APIS_REFRESH_TOKEN=$(gcloud auth print-refresh-token)