Limited console output

As discussed in [this](https://groups.google.com/forum/?fromgroups=#!topic/calabash-ios/X7gyincrq30) google groups thread, here is a way to limit the amount of console output  when running cucumber. Coming from a web-testing background I found the logging out of calabash diagnostic information (timeout data, version numbers, app build directory location etc) to be mostly unnecessary in the normal case where you simply want to see regular cucumber output; i.e. scenarios and steps, and passes and fails. 

The solution I propose here is to set an environment variable prior to running the cucumber tool to specify how much information should be included in the output. For example:
`export CALABASH_FULL_CONSOLE_OUTPUT=false`
will limit the output from cucumber to something similar to this:
```
Feature: User login
	As an normal user
	I want to login
	So I can use the app

	Scenario: Successful login
	Given I am on the Login Screen
	When I enter "username" into the "username" input field
	And I enter "correct_password" into the "password" input field
	...
	etc
```

Here the calabash diagnostic information is omitted and only the cucumber results are printed. For new developers coming to the BDD/cucumber methodology on iOS this is a nicer output in my opinion and they are not overwhelmed by verbose console output, and can simply get their heads around how their steps they have been executed. If things don't seem to be working correctly then the full console output can be turned back on:
`export CALABASH_FULL_CONSOLE_OUTPUT=true`

If the environment variable is not set then the cucumber output will be the same as before this pull-request as the default behaviour is `true`.

I've updated the README file to include another step in the Fast Track installation steps entitled "Setup the verbosity of the cucumber output with the CALABASH_FULL_CONSOLE_OUTPUT environment variable". If you feel this is unnecessary in these steps then it could be moved to the wiki, where I think it would also be beneficial to include some more information about how the console output is limited with some examples not present in the README. Let me know your thoughts and I'll add the information to the applicable page or create a new one.