Optimize ezcDbHandler->quoteIdentifier()

quoteIdentifier is called lots of time during query building*,
this change tries to optimize how many function calls are done
and what kind of calls are done in the most common cases.

* profiling has reveled that this is a noticeable hot spot in the
  code in our case. 
Merge pull request #1 from andrerom/patch-1

Optimize ezcDbHandler->quoteIdentifier()