Enable Travis CI

- Add badge to readme
- Use language 'cpp' because 'csharp' would bring in Mono
- Set to Trusty Tahr (Ubuntu 14.04)
- Only clone necessary submodules
- Use SSH for powershell-ci user access
- Install dotnet-cli
- Build PowerShell
- Run Pester tests

.travis.yml

@@ -0,0 +1,16 @@
+language: cpp
+sudo: required
+dist: trusty
+git:
+  submodules: false
+before_install:
+  - git config --global url.git@github.com:.insteadOf https://github.com/
+  - git submodule update --init --recursive -- src/libpsl-native src/monad src/windows-build test/Pester
+  - sudo sh -c 'echo "deb [arch=amd64] http://apt-mo.trafficmanager.net/repos/dotnet/ trusty main" > /etc/apt/sources.list.d/dotnetdev.list'
+  - sudo apt-key adv --keyserver apt-mo.trafficmanager.net --recv-keys 417A0893
+  - sudo apt-get -qq update
+  - sudo apt-get install -y dotnet-nightly
+script:
+  - dotnet restore
+  - ./build.sh
+  - ./pester.sh