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.
copy receiver stats in Stats.Sub()
add some guards for nil pointers
review tweaks
Merge pull request #212 from Shopify/stats_sub

Improve Stats.Sub()
drop the *s guard
review tweaks
Merge pull request #213 from Shopify/stats_sub2

Drop the *s guard in Stats.Sub()
Minor fix.