Merge pull request #90 from texcoffier/master

Negative string indexes now works: string[-1]
fixed setting class attribute from an instance (now updates the variable on the instance, not the class)
javascript mode: fixed calling function that takes keyword arguments where caller omits the names.
release 0.9.1
fixed bug in keyword defaults in javascript mode.
new ternary logic speeds up "+" addition/concatenate operator (by checking if the first variable is a number)
added PyPy to regression tests/benchmarks
release version 0.9.2
`sum` builtin, and new benchmark testing float performance.
fixed bug in lua backend.
new optimization for '+' operator to bypass ternary/__add_op check `pythonjs.configure(direct_operator="+")`
fixed __getslice__ for dart backend.
fixed bug in dart backend with `list()`
fixed bug in lua backend, for loop over a list.
lua backend: fixed `[None]*n`
lua backend: fixed augassignment `a += 1`
fixed bugs in the dart and lua backend.
optimized call overhead of normal js backend.
refactoring translator
dart backend: fixed null strings `"\0"` and list comprehensions.
dart backend: fixed null list slice `a[:]`
dart backend: fixed printing double numbers - `a="%s"%1.1`
made benchmark tests dart compatible and optimized them.
lua backend: fixed tuple return from function.
dart backend: fixed destructured assignment `a,b = c`
added nbody benchmark