Fix memory leak [Fixes #32]

Instead of keeping track of a global list of all exceptions, instead
store the necessary context on the exception itself.
Don't inifinite-loop on re-raise
Improve loop example
Fix rspec example
Start of fix for #31; needs work
Merge branch 'handle-syntax-errors'
Rely on nil check in Pry.rescued [Closes #31]

Post recent refactoring, it should not be possible for
enter_exception_context to be called with an invalid exception.

`rake sintax` now does this: http://showterm.io/b4b797cf4e4676223936e
Support capybara/rspec better [Fixes #28]
v1.1.0