Add default User-Agent header

Added a default User-Agent header for HTTP requests in download cache.
Modified existing tests to use this header.
Let the User-Agent header be modified from the env vars

Users can now modify the User-Agent header from the manifest file.
Merge pull request #125 from spaungam/stagingoutput

add compile begin and end markers
Fix rubocop issues

Mainly whitespace issues, plus an outdated hash syntax.
Merge pull request #126 from ThomWright/user-agent-fix

User agent fix
Use different User-Agent in RSpec HTTP requests

RSpecs that send actual HTTP requests to download assets should
use a non-default User-Agent to make it clear the requests are
coming from tests.
Standardize and Clean CLI output
Show full URL if downloading file, otherwise show filename.
Merge pull request #127 from svennam92/simplify-output

Standardize and Clean CLI output
Merge branch 'master' into user-agent-fix

Conflicts:
	spec/bin/compile_spec.rb

Sync'ed my fork with the upstream repo.
Merge pull request #128 from ThomWright/user-agent-fix

User agent fix - update to RSpecs