Structure: added IDatabaseStructure implementation
Conventions: added IConventions implementation
Selection, SqlBuilder and Context: use IConventions (BC break!)

Deprecated:
- Nette\Database\IReflection
- Nette\Database\Reflection\ConventionalReflection
- Nette\Database\Reflection\DiscoveredReflection
- Nette\Database\Reflection\MissingReferenceException
- Nette\Database\Reflection\AmbiguousReferenceKeyException
ActiveRow & Selection: moved loading to Selection
SqlBuilder: fixed support when IConventions does not found reference
DI: added support for IConventions
Context: added IDatabaseStructure, Selection uses cached version of getColumns() [closes nette/nette#1360]
Updated tests for IConventions
phpdoc typos