cache require('nw.gui').App.dataPath

Signed-off-by: Niv Sardi <xaiki@evilgiggle.com>
make API_ENDPOINT configurable via POPCORN_API env variable,

run with env POPCORN_API=http://localhost:5000 nw . --debug to test

Signed-off-by: Niv Sardi <xaiki@evilgiggle.com>
Merge pull request #188 from xaiki/dev-0.3

database nitpicks