Remove 'expensive' filters
Remove NestedData
Remove Log, add Logln

Log allowed callers to specify a custom source path, which won't work once we're
getting caller information from their PC.  Remove it, and use Logln instead.
Filter on caller's PC rather than logger name

Overrides can be specified in one of three formats, in order of increasing
specificity:

  * "path/to/pkg" filters based on the Go package path of the logger.
  * "path/to/pkg/filename.go" filters based on the filename of the logger.
  * "path/to/pkg/filename.go:123" filters based on filename and line number of
    the logger.

When filtering a log record, the most specific override wins.
Add TODO to shouldLog
Merge pull request #13 from kevinwallace/pc_filter

Filter log records based on caller's PC, rather than nested logger name