[TASK] Implement trustworthy field name, improve checks and resulting messages
[TASK] Cache instantiated Providers in ConfigurationService

Should save a few seconds off here and there, especially for ConfigurationProviders which also themselves implement internal caching.
[BUGFIX] Perform double ConfigurationProvider registration

Some pages, when rendered in FE, will not load the ext_tables.php files or will prematurely output content (if no USER_INT are used). To cover this case we need a double registration from ext_localconf.php as well (FE will use it always, BE sometimes. BE will always use ext_tables.php, FE will sometimes - so double necessary).
Merge pull request #66 from NamelessCoder/bugfixes

[BUGFIX] Fix case of missing ConfigurationProvider registration and some resolve-related issues