add svn support

[jboverfelt]

Adding basic support for dependencies located in SVN repositories.

[petejkim]

Could you provide a publicly accessible subversion repo that I can test this on? (sorry)

[jboverfelt]

Here's one that I made on google code to test this:

https://code.google.com/p/golang-svn-test/source/checkout

You'll have to specify the import path as

code.google.com/p/golang-svn-test/trunk

since SVN divides up trunk/branches/tags.

Apologies for the dummy repo - the real reason for this change is to support some closed source repositories that I work on. I have a change I'll submit soon for bzr support, which has a nice public test case in the mgo (a Go MongoDB library)

[jboverfelt]

One more thing - this dummy repo has 3 revisions so you can specify

code.google.com/p/golang-svn-test/trunk #2 

To test that it handles older revisions correctly.