Add Dockerfile for testing (braintree#160)

# What

Add a Dockerfile containing setup for repeatable local development on cpairs.

# Why

Setting a cpair up to develop on server SDKs is tedious. The instance has to be setup, and the setup instructions in the [internal wiki](https://internal.braintreepayments.com/display/DEV/Client+Library+Development+Tips) don't work verbatim on cpairs, so it's a time consuming activity to get setup. Instead of assuming cpairs are setup or expecting developers to manually set them up, lets move that into a Dockerfile so it is predictable and documented in code.

# Example

Run gateway on cpair:
```
drake exec --target authy dbdo
drake exec --target provisioner dbdo
drake exec --target arbiter dbdo
drake exec --target integrations dbdo
drake exec dbdo
drake exec rake db:populate_cosmos_data
drake up -d authy
drake up -d provisioner
drake up -d arbiter
drake up -d integrations
drake up
```

Open console in docker container and run make
```
make
# rake
```

Dockerfile

@@ -0,0 +1,13 @@
+FROM debian:wheezy
+
+RUN apt-get update
+
+# For installing hhvm
+RUN apt-get install -y apt-transport-https software-properties-common
+RUN apt-key adv --recv-keys --keyserver hkp://keyserver.ubuntu.com:80 0xB4112585D386EB94
+RUN echo deb https://dl.hhvm.com/debian wheezy main > /etc/apt/sources.list.d/hhvm.list
+RUN apt-get update
+
+RUN apt-get -y install curl rake php5 php5-cli php5-curl php-pear hhvm phpunit
+
+WORKDIR /braintree-php

Makefile

@@ -0,0 +1,10 @@
+.PHONY: console build
+
+console: build
+	docker run -it -v="$(PWD):/braintree-php" --net="host" braintree-php /bin/bash -l -c "\
+		curl -sS https://getcomposer.org/installer | php -d suhosin.executor.include.whitelist=phar && \
+		php -d suhosin.executor.include.whitelist=phar ./composer.phar install; \
+		bash"
+
+build:
+	docker build -t braintree-php .

README.md

@@ -83,6 +83,14 @@ Version [2.40.0](https://github.com/braintree/braintree_php/releases/tag/2.40.0)
 
  * [Official documentation](https://developers.braintreepayments.com/php/sdk/server/overview)
 
+## Developing (Docker)
+
+The `Makefile` and `Dockerfile` will build an image containing the dependencies and drop you to a terminal where you can run tests.
+
+```
+make
+```
+
 ## Testing
 
 The unit specs can be run by anyone on any system, but the integration specs are meant to be run against a local development server of our gateway code. These integration specs are not meant for public consumption and will likely fail if run on your system. To run unit tests use rake: `rake test:unit`.