Add query transformation callback feature

Implementation of query transformation requested in ticket #66:
* Add query_transformation.h with internal types
* Add method session::set_query_transformation<T>() - assigns
user-defined function or functor
* Add session::get_query() - returns string with transformed query.
* Replace use of session::get_query_stream() with get_query()
* Keep session::get_query_stream() to access original query accumulated
in current session statement
* Add test_query_transformation to common tests.
Tested using VS2012 on Windows 8 against DB2, ODBC (PostgreSQL, SQL
Server), MySQL, Oracle, SQLite3.
Use const& for std::unary_function Arg parameter

Clear TODO item for query_transformation feature (ticket #66)
Merge pull request #82 from mloskot/issue-66

Implementation of query transformation callback feature
Move macros and ensure_approx function.

Allow use of ensure_approx as free function.
Forgot to clear TODO comment
Comment on purpose of query transformation
Add state to functor in query transformation test

Change functor with hard-wired test to one with configurable WHERE
condition.
Use NULL consistently, instead of Zero
Null-initialize session members for pool-mode
Split .travis.yml actions into shell scripts

Travis CI recommends to keep all extensive steps in shell scripts and
keep .travis.yml minimal.
It is easy to syntactically break .yml file.
Make Travis .sh scripts executable
Attempt to install oracle-xe-client on travis-ci

Testing Oracle setup for the first time. Temporarily disabled ctest run.
Use Bash shell to run travis-ci scripts

Fix echo call
Fix typo in Bash function tmstamp
Install Oracle GPG key for apt-get on travis-ci
Set ORACLE_HOME for InstantClient on travis-ci

Add newline at end of .sh files
Export ORACLE_HOME to global env on travis-ci. Fix broken apt-key.
Check ORACLE_HOME from script.sh builder
Still testing ORACLE_HOME export

Not sure why environment variable export is not working as expected.
Move common definitions to common.sh

Source common.sh into other travis-ci shell scripts.