Initial MSSQL related commit.
Merge branch 'master' of github.com:yiisoft/yii2 into mssql

Conflicts:
	yii/db/Connection.php
MSSQL driver classes moved to the proper location.
MSSQL table metadata parsint (initial version).
MSSQL Schema: methods order changed, initial data type infer.
Initial MSSQL tests.
Merge branch 'master' of github.com:yiisoft/yii2 into mssql
Merge upstream/master into upstream/mssql.
Refinements to MSSQL driver classes.
MSSQL initial query builder.
MSSQL AR tests.
Merge pull request #351 from yiisoft/mssql

Initial MSSQL support.