grab full bins for diff in parallel
fix for parallel diff generation
tests to ensure PX dyno scale works
show dyno size in hk dynos

Fixes #72.
fix ensureSuffix
fix arch detection for linux users

Fixes #74.
Revert user.Current().HomeDir to os.Getenv("HOME")

user.Current() requires cgo and thus doesn't work with cross-compiling.
This appears to be how the Heroku Toolbelt works, though per @fdr it may
not be correct for all cases (when users have modified their home dir).

We will address those issues if/when they come up.

Also moved these functions to util.go