Adding configurations for AppVeyor and Travis CI

.travis.yml

+language: csharp
+solution: DocX.sln
+install:
+  - nuget restore DocX.sln
+  - nuget install NUnit.Console -Version 3.4.1 -OutputDirectory testrunner
+script:
+  - xbuild /p:Configuration=Debug DocX.sln
+  - mono --debug ./testrunner/NUnit.ConsoleRunner.3.4.1/tools/nunit3-console.exe ./UnitTests/bin/Debug/UnitTests.dll

appveyor.yml

+before_build:
+  - nuget restore DocX.sln
+build:
+  project: DocX.sln
+  verbosity: minimal