Added callback to README

Implemented download progress callback

Fixed integer overflow of filesize

Renamed constants

update docs to show how to handle common executable not found error

update gitignore and modify test

add some test and update documentation

Update README.md

Update README.md

Solved a small issue in the test

Merge pull request #1 from lepouletsuisse/master

Add possibility to set executable path

Update pom.xml

Better documentations

Solve issue where multiple options command failed to execute