allow changing the log level at runtime

Install a signal handler when passing :change_at_runtime => <SIG>
to use_log_level_option to signal <SIG> and toggle the log level
when receiving this signal.

Implements #30
fix typo
try to fix travis build

increase sleep after sending signal
fix JRuby builds

JRuby has its own handler for USR1 so use USR2 in the tests
refactor runtime toggling of the log level

* change argument name
* make toggle_log_level private
* simplify argument handling
* refactor and adjust tests
Merge remote-tracking branch 'sch0rsch/change_log_level_at_runtime'
make new toggle available in scaffolded apps