Bugfixes: del-, try-except-, and import-as-syntax

While using codegen I noticed that the following code examples could not be parsed to an ast and then translated back again to source code with codegen:

```python
# example 1
del l[0]
# example 2
del obj.x
# example 3
try:
    '#'[2]
except IndexError:
    print 'What did you expect?!'
# example 4
try:
      l = []
      l[1]
  except IndexError, index_error:
      print index_error
# example 5
import intertools as iterators
# example 6
from math import floor as fl, ceil as cl
```

So I forked the code, fixed these issues and now I'd like to contribute my fixes to the actual repository.