Added basic RapidViews(Agile boards) support

Added basic support for JIRA Agile boards.

Usage:
```ruby
client = JIRA::Client.new(options)
# Get all rapid boards
boards = client.RapidView.all
# Or find one
board = client.RapidView.find(1)
# Get all issues related to a board
issues = board.issues
```

Probably will also need to add descriptive error messages when trying to get boards from non-agile JIRA instance.