0.1.4
Use bin-wrapper to download, test and build pngquant
build-essential is already installed
`bin.path` should always return the local binary in tests
Add note about Linux dependencies

Fixes #2.
Update OS X bin to pngquant 2.0.1

which isn't tagged on github, so we can't update for all platforms yet
0.1.5
Add quotes for cases where there are spaces in path
Update Linux x64 binary to version 2.0.0
Fix Windows URL

Fixes #13.