Define RARRAY_AREF for Ruby 2.0 compatibility.
Merge pull request #20 from vjoel/ruby-2-0-compat

Define RARRAY_AREF for Ruby 2.0 compatibility.
Fix #12.
Readonly transaction can keep GVL in mdb_txn_begin.
Move TxnArgs to header file.
Handle interrupts accumulated during wait for mdb write mutex.
Merge branch 'fix-issue-12' of github.com:vjoel/lmdb into fix-issue-12