Add Appveyor to check build integrity on Windows machines

[dfalster]

We already have travis enabled, better add ensure works on windows too!
https://ci.appveyor.com/

[dfalster]

Argh, failed again, similar issue. Doesn't like the remake::install_missing_packages() step:

Rscript -e 'remake::install_remake(".")'
[1] "remake::install_remake(.)" 
Rscript -e 'options(repos="http://cran.rstudio.com"); remake::install_missing_packages();'
Error: unexpected end of input
Execution halted
Command exited with code 1