Stop defining GOPATH in build script.

This makes it possible to install the dependencies in
people's workspaces rather than inside our repository, and
integrates better with the go tool.

The next step would be to split the server in its own
repository with *.go files at the root so one can just do:

go get github.com/dougt/whereismyfox-server
go build github.com/dougt/whereismyfox-server

and import packages from this repository as usual.
Remove some misplaced calls to Writes() when building API
Server-side changes for commands.

The server stores the recognized commands in a 'commands' table,
and keeps track of which devices implement which commands in a
'device_has_command' table.

The 'commands' table is initialized when the server is started
by reading the commands.json file.

The server issues a push notification for the device to trigger
a command and saves the context of the command (that is, command and
arguments) in memory waiting for the device to fetch it.

The will device the command ids it supports to the server every time
the app starts up.
Support for commands in the app.
Basic support for commands in the web ui.
Merge pull request #13 from dougt/commands

Generic support for commands