Add DefaultOptions variable.

This commit adds an explicit DefaultOptions variable for additional documentation.
Open() can still be passed a nil options which will cause options to be change to
the DefaultOptions variable. This change also allows options to be set globally for
an application if more than one database is being opened in a process.

This commit also moves all errors to errors.go so that the godoc groups them together.
Merge pull request #210 from benbjohnson/default-options

Add DefaultOptions variable.