Add dynamic pool creation via pooler:new_pool/1

Mostly for testing purposes, also added pooler:rm_pool/1.

Updating overview doc, added doc for new_pool (documenting pool config
proplist) and adding basic test coverage for dynamic pools.
Update copyright year in pooler_starter.erl
Allow pooler application start without pools config key

The pools key is now optional since pools can be entirely configured
at run time via pooler:new_pool/1.

Improved unit test coverage for dynamically created pools and coverage
for app start without pools config key.
Update README with example of using pooler:new_pool