Add context handling & warnings

Added a warning when methods are added to Selenium::Webdriver to encourage people to log issues.

Added context handling:

`current_context` & `current_context=` get and set current context.
`available_contexts` returns an array containing available contexts
`switch_to_default_context` does as told
`within_context` executes a block in the context given, then switches back to the context used before the block executed.