linking against our readline fails on ubuntu 11.10

[JeffBezanson]

On linux we should use the system readline.

[JeffBezanson]

Also we'll need to add release notes on some extra packages that need to be installed on ubuntu (probably involving the readline headers). We can also add instructions for installing readline on mac.

[ViralBShah]

We could set up the build so that libreadline is used on linux. On mac we should retain downloading and building of readline.

[ViralBShah]

For ubuntu, it may be worthwhile to also build a .deb package, which is not too difficult. The package can be hosted on beowulf, and people can just add it to their apt sources.

[ViralBShah]

Did anyone track down why it was failing?

[JeffBezanson]

Now documented in readme.