(SERVER-112) Move puppet/puppetserver ruby config out of `master.rb`

`master.rb` was getting pretty hairy.  This commit moves the code
related to initializing Puppet Server's ruby stuff into the
Puppet::Server::Config class, and introduces a new class
Puppet::Server::PuppetConfig to house the Puppet ruby initialization.
(SERVER-112) Move `execution` code into its own class

The `Execution` stuff seemed out of place in `master.rb` and
`config.rb`, so this moves it to its own spot.
(SERVER-112) Move request handler logic to module

There was a lot of logic related to the Puppet::Network::HTTP::Handler
module that was inlined in `master.rb`.  This commit simply moves
it into its own module that we can mix in instead of mixing in
the one from Puppet directly.
(SERVER-112) Move `run_mode` to end of file

The `run_mode` method in `master.rb` was defined in the middle
of the methods that satisfy the `JRubyPuppet` Java interface.
This commit simply moves the method down so that all of the
methods related to that interface are grouped together.