get_string: Refactoring, performance improvements, bug fixes and unit tests

get_string: Refactoring, performance improvements, bug fixes and unit tests

MDL-18669 get_string refactored to elimiate duplicate code and make it easier to understand.
MDL-17763 parent language not processed correctly when getting a plugin string.
MDL-16181 more intelligent caching to avoid repeated file_exists checks.
MDL-12434 move values to array keys to improve lookup times.

The main part of the refactoring is to create a singleton string_manager class to encapsulate the cached data and the processing, while breaking the code up into more smaller methods.

Other performance improvements include:
* Cache results of plugin name -> locations to search array.
* Cache parent lang lookup.
* Skip eval if the string does not contain $ \ or %.
* Remove the unnecessary sprintf from the eval.

There is a performance testing script in lib/simpletest/getstringperformancetester.php. For now this script has the old get_string implementation copied and pasted to the end, and renamed to old_get_string to allow for comparitive timings.

There are now some unit tests for get_string in lib/simpletest/teststringmanager.php. I think I have managed to cover most of the tricky cases.