Dump go get -u -d output as a string rather than byte slice.

Addresses one point discovered in #5.
Add debug nil pointer checking to updateHandler.

Should help me figure out what went wrong in #4, if I can reproduce it.
Add basic support for getting changes in "gopkg.in/" packages.

Need to create a general interface to allow more package sources to
have implementation for getting changes.
Ensure the Update button becomes completely disabled after click.

Partially addresses problem in #5.
Work on support of keeping track packages being updated.

This will show the update button as "Updating..." even if the user
refreshes the page while the package is not yet finished updating.