LogFormatter improvements

[tburry]

The LogFormatter right now is hardcoded to php://output. I want to be able to specify another output stream or even multiple output destinations, some of which may not be a file stream (ex. database, syslog). The garden-cli won't be responsible for doing any of the work or wiring these up; it just needs to make useful classes available to libraries.

Any changes must be backwards compatible.

[cdepage]

"Done" criteria (after lengthy discussion with Todd & dePage):

• interface for "formatters" to implement
• stream writer
• collection writer
• PSR writer