Refactor config and use account-level push api key

Refactor and simplify the usage of the config class and switch to using
a single push api key for all environments. Also includes some general
cleanup.
Slightly refactor transaction formatter

Remove to_appsignal_hash and remove redundant part of name.
Move capistrano classes to integrations
Move Railtie to integrations
Change config setting to attr_accessor
Namespace classes properly
Enable development by default and comments in config
Merge pull request #98 from appsignal/one_push_key

[MERGE] One push key
Less blocking thread safety

Use Thread.exclusive and thread_safe gem to lock as little as possible.
The gem now only locks on access to the queue array and when replacing
the aggregator when it is to be sent. Truncating and so on are now
outside of the lock.
Merge pull request #100 from appsignal/thread_safe

Less blocking thread safety
:fish:
Switch rails dependency to activesupport
Testing setup for different combinations of dependencies
Make rails-independent
Merge pull request #101 from appsignal/no_rails

No rails required
Set sleep_time to 10 in development mode
Remove jruby from Travis allowed failures
Add rbx-2.1.1 and jruby-19mode, remove 1.8.7
Keep log directory in project_fixture
Sort list of present environments

So the spec doesn't fail on Linux.