Expose keep_build to the command line

This allows the user to specify the keep_build argument, thus whether the build
directory should be always kept, never kept (currently the default) or only
kept on error.
Update tests for the new keep_build argument

The build() method does not have any default arguments, so we continue in this
convention and simply require all code (including tests) to explicitly specify
keep_build.
Merge pull request #139 from hashdist/keep_build

Expose keep_build to the command line