Add Sprockets::Helpers.debug accessor

When Sprockets::Helpers.debug is set true,

     :expand   => true
     :digest   => false
     :manifest => false

will be set automatically.
This is useful for application development mode.
Add debug mode explanation to README.
Merge pull request #15 from YasuOza/feature/global_debug_mode

Feature/global debug mode
Expand assets with pretty output
Merge pull request #16 from YasuOza/better_expand_output

Expand assets with pretty output