Merge pull request #6 from ldecicco-USGS/master

Added appveyor.yml file.

appveyor.yml

+init:
+ps: |
+$ErrorActionPreference = "Stop"
+Invoke-WebRequest http://raw.github.com/krlmlr/r-appveyor/master/scripts/appveyor-tool.ps1 -OutFile "..\appveyor-tool.ps1"
+Import-Module '..\appveyor-tool.ps1'
+install:
+ps: Bootstrap
+build_script:
+- travis-tool.sh install_deps
+test_script:
+- travis-tool.sh run_tests
+on_failure:
+- travis-tool.sh dump_logs
+artifacts:
+- path: '*.Rcheck\**\*.log'
+name: Logs
+- path: '*.Rcheck\**\*.out'
+name: Logs
+- path: '*.Rcheck\**\*.fail'
+name: Logs
+- path: '*.Rcheck\**\*.Rout'
+name: Logs
+- path: '\*_*.tar.gz'
+name: Bits
+- path: '\*_*.zip'
+name: Bits
\ No newline at end of file