Add configuration class, ability to specify a branch to run coverage on, and ability to silence warnings

* Allow configuration of a branch to report coverage on (all is the default)
* Allow disable of warnings ("Not reporting to Code Climate..." - showing warnings is the default)

Since Code Climate only runs analysis on one branch, it seemed reasonable to be able to specify a branch for the coverage as well.

Configuration is a pretty standard setup (and obviously needs to be called before start):

**with a block:**
```ruby
CodeClimate::TestReporter.configure do |config|
  config.branch = :master                      
  config.show_warnings = false                 
end                                                         
CodeClimate::TestReporter.start
```

**individual lines:**
```ruby
CodeClimate::TestReporter.configuration.branch = :master
CodeClimate::TestReporter.configuration.show_warnings = false
CodeClimate::TestReporter.start
```