Fix error when setting value to null.

clearing the CustomURL didnt work, so once you have set a customURL, it is there for good and cannot be cleared. Needed to use strict checking on exists() to solve the problem
Merge pull request #10 from Leapfrognz/patch-1

Fix error when setting value to null.
Fixed isssue with PageId set to 0
Merge commit '7601a26a3bde0bedcffade70f0a83f47693946cb'

# By Zauberfisch
# Via Zauberfisch
* commit '7601a26a3bde0bedcffade70f0a83f47693946cb':
  Do not run 3.0 unit tests as this branch is now 3.1 only
  fixed visbility of statics and methods according to 3.1 rules / conventions

Conflicts:
	code/LinkField.php