Rename public stats attributes
Combine PStatGroup and PStatLocation

PStatGroup was never used on its own.
Remove unused `tree` attribute
Remove BaseStats base class

The methods defined there were unused
Make utility function public
Improve repr output
Remove unused methods
Add test of PStatsLoader
Fix commented out code

Note that the CLI still doesn't properly initialize the app.
Rename for clarity
Rename objects to match graph terminology

- PStatGroup to PStatsForest
- PStatRow to PStatsNode
Factor out raw_stats_to_nodes function
Rename callers to _callers for privacy and to prevent confusion

There really shouldn't be an `obj.caller` and an `obj.callers`.
Change tree to point to the slowest tree.

`PstatsLoader.forest` points to all known trees.
Remove PStatsForest which was never really used

The profile uploader only used the slowest tree.
Factor out test utilities and add tests for upload module
Factor out node_attrs function from `stats_to_tree_dict`
Minor reformatting of html template
Switch from optparse to argparse
Limit the depth of profile trees