implemented ALTER TABLE DROP COLUMN

git-svn-id: http://sphinxsearch.googlecode.com/svn/trunk@4445 8b96e2b9-35c5-2c16-bc47-5122d61876d4
fixed test_213 model

git-svn-id: http://sphinxsearch.googlecode.com/svn/trunk@4446 8b96e2b9-35c5-2c16-bc47-5122d61876d4
merged rel21 branch (up to r4449) back into trunk

git-svn-id: http://sphinxsearch.googlecode.com/svn/trunk@4450 8b96e2b9-35c5-2c16-bc47-5122d61876d4