• constant_inc.php: add ANYBODY and NOBODY to the access levels and replaces spaces with tabs everywhere
• lang/strings_english.txt: new error for missing custom field
• core/custom_field_api.php: new API file (in progress)

Commit intial work on custom field api.

Christian got the ball rolling, Victor and I reviewed it through several iterations. Though my intention was to simply do a quick review and check this in, doing so made me think about several things and I made some changes along the way. We can carry on from here as a base point.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1618 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9