Fix leak in complete() callback of Database.each()

Add destructor to EachBaton that disposes the `completed` callback function.
avoid duplicate package [skip ci]
avoid duplicate package [skip ci]
AppVeyor: `echo %PLATFORM%`
Chocolatey install `wget`, `7zip.commandline`; get&unpack node-webkit
the previous commit message used an ampersand and thus line 31 failed
unset `nw_version` on x64, `npm install nw-gyp`, build, test, publish
the most recent node-pre-gyp's tarball (uses mapbox/node-pre-gyp#95)
[skip ci] a command file that would build sqlite3 for node-webkit
keep node-webkit-related commands in a separate script
move more logic `appveyor.yml` → `scripts\build_for_node_webkit.cmd`
node-webkit v0.10.0 is released → start building for it on Travis CI
print the environment when `scripts/build_for_node_webkit.cmd` starts
explicitly pass `%PLATFORM%` to `scripts/build_for_node_webkit.cmd`
output the given platform from `scripts/build_for_node_webkit.cmd`
environment: copy `PLATFORM` to `NW_PLATFORM`
node-webkit 0.10.0 is released and we shall build for it
make a matrix of TARGET_ARCH, build on Linux 32-bit for node-webkit
upgrade `node-pre-gyp`, using the tarball of its `master` branch
add `PLATFORM` variable to the matrix (as @FeodorFitsner recommended)