Added --no-index option to install and bundle command which temporarily disables querying the package index

Sort requirement sources to make sure the choice isn't made because of the order in which the links are found but what is useful.

HTMLPage.base_url was being called excessively since it is used as a property
and called on every link found on the page (if not more).

this has caused quite a bit of confusion by Pinax users thinking the 404s are
errors. in most cases the 404s are harmless and don't prevent the installation
or location of a package.

if there is cause for concern in this i recommend to output something once
*all* pages have been hit and all produced some sort of error.

Integrated download feature with RequirementSet, simplified archive creation in an own method of InstallRequirement. Added ability to the version control support classes to export a repository.

Fixed #19 - Added support for base URLs using <base href='...'> when parsing HTML pages. Thanks filip.

Fixed #40 - make sure the branch name is correctly looked up when freezing an editable packages from a Git repository