Add Environment, Driver, Connection, Statement

vapi file:

- Add various enums and types necessary for the following functions
- Add SQLAllocHandle function
- Add SQLSetEnvAttr function
- Add SQLDrivers function
- Add SQLDriverConnect function
- Add SQLExecDirect function
- Add SQLNumResultCols function
- Add SQLGetDiagRec function

vala file:

- Add UnixOdbcError errordomain
- Add Environment class (basic ODBC environment)
- Add Driver class (name and attributes of an installed driver)
- Add Connection class (connection to a database)
- Add Statement class (SQL statement)