Fix to issue #323

See for more details:
https://github.com/markrendle/Simple.Data/issues/323
Solves Casting String to GUID

When retrieving a string from the database and trying to map it to a
GUID the safe convert would throw an exception and return null.  This
allows for converting strings to GUIDS if possible.
Adding Schema Support

This commit adds schema support to Simple.Data.Ado providers via the new
ISchemaConnectionProvider interface, and allows them to support a schema
per instance of Database.Open(...).  This is primarily used for Oracle
databases.  In order to use the schema, just add the schema name to the
appropriate Database.Open call (e.g.
Database.OpenNamedConnection("TestConnectionName", "TEST_SCHEMA_NAME")
).
Merge pull request #324 from jdscolam/master

Fix to issues #306 and #323