Add default User-Agent header

Added a default User-Agent header for HTTP requests in download cache.
Modified existing tests to use this header.
make common_paths available
remove a whitespace
Documentation update
Merge branch 'master' into commonpaths
correct a unit test and minor doc update
Disable tests for Ruby 1.9.2
Merge pull request #123 from spaungam/commonpaths

Commonpaths
Let the User-Agent header be modified from the env vars

Users can now modify the User-Agent header from the manifest file.
use https
Merge pull request #124 from jgawor/https

use https
add compile begin and end markers
remove detect output
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Mainly whitespace issues, plus an outdated hash syntax.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