Update installation documentation links.

This PR updates some of the links in (mainly) the installation notes. In particular,
1) Links to EPD are now replaced with links to Canopy.
2) Links to `svn.enthought.com` are replaced with something that exists, usually `docs.enthought.com`.
I just removed the references to `svn.enthought.com/enthought/wiki/Install` as there did not seem to be anything to replace this with. I also removed the link to IntelMacPython25 build notes, as this also has no current replacement and is thought to be obsolete.

Please note that I did not try really update the installation notes, this PR just attempts to fix links that are either broken or deprecated, and leaves it at that.

@cgodshall : This should make the changes you desire.