Separate responsibilities of Adhearsion.active_calls into a passive collection and an active supervisor

This allows for greater stability (the actor portion is a simple supervisor which is crash-resistant) and greater performance (call lookups are now synchronised by a simple Monitor, rather than an actor, which is much faster).
Merge branch 'refs/heads/develop' into feature/passive_calls_collection
[BUGFIX] Specs expect active_calls itself to be an actor
Merge branch 'refs/heads/develop' into feature/passive_calls_collection
Merge branch 'feature/passive_calls_collection' into develop
Bump the min rspec development dependency to 2.13
Merge pull request #468 from sfgeorge/feature/update-min-rspec

Bump the min rspec development dependency to 2.13
[BUGFIX] Plugins in app lib directory should be loaded for their rake tasks
[BUGFIX] Plugins should be more comprehensively loaded for rake tasks
[BUGFIX] Fix a spec on JRuby
[BUGFIX] Ensure that millisecond logging occurs in default file log
[CS] DRY-up sharable logging options for file and stdout
Add millisecond file log entry to CHANGELOG
Merge pull request #474 from sfgeorge/feature/millisec-file-logging

[bugfix] millisec file logging
[UPDATE] Bump to 2.5.1
Merge branch 'release/2.5.1' into develop
Note that 2.5.1 has been yanked

@bklang is unhappy with the fixes to outbound calls which require corresponding fixes to the Rayo server being included in a patch release since they are backward incompatible. They need to either be made backward compatible before a re-release (preferable) or delayed until 2.6.0
handlers can be registered with additional options
Merge pull request #475 from christophersansone/register_handler_with_options

[BUGFIX] Call handlers can be registered with additional options
[DOC] Fix broken record beep docs

Fixes #480