Directly consume options from Ec2ServerCreate & Bootstrap plugins.

This should help to bring in all relavent options from these 2 plugins
rather than having to duplicate the configuration in multiple places.

The tricky implementation details of this are in the `deps` block. Since
this block is called when the plugin's #load_deps method is called any
class-defined options will have already been added to the `self.options`
Hash. This is the high level idea:

1. Options from Knife::ServerBootstrapBase will be loaded via the
  `include` line.
2. Local options will be loaded next, possibly overriding values from
   Knife::ServerBootstrapBase.
3. At a later time `#load_deps` is called on the object which will:
   1. Save the current options Hash in a variable for the moment.
   2. Load the options Hash from Knife::Bootstrap (or
      Knife::Ec2ServerCreate) replacing the object's options Hash.
   3. Merge back to original options Hash ontop of the current options
      Hash so that the options from Knife::Bootstrap have the lowest
      priority over common and local options that have already been
      defined.

In this way, the priority of options being loaded from lowest to highest
are:

1. Underlying Knife plugin such as Knife::Bootstrap
2. Common options from Knife::ServerBootstrapBase
3. Plugin-specific options from the class itself
Ensure config parameters are applied in the right order for Chef 10/11.

In addtion, 2 parameters have better support:

* Add support for the `--host-key-verify` flag in all SSH commands
* Pass the value of `config[:verbosity]` to the downstream bootstraping
  Knife plugins.

Updates to Knife by way of Chef 11 fix the error-prone behavior of
loading configuration into a Knife plugin. Details are on the Docs
website at:

http://docs.opscode.com/breaking_changes_chef_11.html#knife-configuration-parameter-changes
Update README badges.
[Breaking] Make knife-ec2 an optional gem dependency.

This was done to keep the dependencies of this gem lighter and to make
future adapters easier to add (using the same mechanism of lazy
loading).

Listing the Knife subcommands will still behave as normal and `knife
server boostrap ec2` will still appear. However if the knife-ec2 is not
present when a user attempts to use the subcommand (or display its full
help), a warning message will be displayed noting this requirement and
the subcommand will terminate with a non zero exit code.
Add matrix build support to TravisCI for multiple versions of Chef.