Improve versioning for builds.

By default, this project uses a git describe for versioning. This
doesn't work very well when you have a source tarball (without a .git
directory). So when generating a tarball, a VERSION file will be
written.

If that VERSION file is present, it will be used the VERSION when the
build target is called via make. This should help with Homebrew
building.
Add a .gitignore for generated artifacts
Update the homebrew formula to have a versioned artifact

Prior to this there was a bug in the homebrew tarball formula that would
make the version of the vmpool binary be nil and show up as clang. Since
the tarball generation is now fixed, and the new tarball is updated,
this is just a simple homebrew formula update.
Makefile adjustments for tarball task

This fixes the tarball task to to work even if the local directory isn't
the exact name of the github repo.
Add simplest basic readme possible
(maint) Refactor subcommands and support retrieval of multiple vms

This commit refactors the subcommand layout to the layout prescribed by
the `go` tool which should make man pages a little easier to navigate.
This commit also adds functionality to support retrieval of multiple vms
using a space delimited list of pool names after the grab command.
(maint) Abstract http requests out of individual commands

This commit abstracts the handling of http requests to a single Request
function which may be handed arbitrary json in string format. Also
improves the validation of the list command. This commit also adds the
ability to delete multiple vms at once by passing them to the delete
command.
(maint) Move utility functions to their own file

This moves the utility functions to their own file and adds some better
error message by telling the user exactly what names were invalid when
failing to fetch vms.
(maint) Update the Makefile to use multiple .go files

This commit builds the executable for vmpool using any file with a .go
ext in the src directory fixing the Makefile to work with the recent
abstraction changes.
(maint) Un-De Morgan the usage check conditional for list command

This commit changes an `||` (or) in the usage check for list to an `&&`
(and) as it should be.
Merge pull request #2 from ajroetker/maint_support_retrieval_of_multiple_vms

(maint) Refactor subcommands and support retrieval of multiple vms
(maint) Update README for multiple vm fetches
Merge pull request #3 from ajroetker/maint_support_retrieval_of_multiple_vms

(maint) Update README for multiple vm fetches
Minor updates to readme
Add note about VMPOOL_URL env var
update gitignore

Gitignore should include the binary artifact. That shouldn't be in
our repo.
Really remove binaries
(maint) Fix spelling on DELETE from DETELE
Merge pull request #4 from ajroetker/master

(maint) Fix spelling on DELETE from DETELE