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

Since pip freeze defaults to outputting to stdout, the default behavior
combined with simple redirection is more than enough to get a requirements
file.