Fix broken test suite

[nicksay]

Currently, running make tests fails with a couple problems:

Traceback (most recent call last):
  File "scripts/crunner.py", line 141, in process_file
    current_output = template.main().encode('utf8')
NameError: global name '_buffer_write' is not defined

and

Traceback (most recent call last):
  File "scripts/crunner.py", line 217, in <module>
    test_runner.process_file(filename)
  File "scripts/crunner.py", line 132, in process_file
    template_module = spitfire.runtime.import_module_symbol(module_name)
  File "/Users/nicksay/dev/spitfire/spitfire/runtime/__init__.py", line 56, in import_module_symbol
    module = __import__(module_name, globals(), locals(), [symbol_name])
  File "/Users/nicksay/dev/spitfire/tests/ambiguous_in.py", line 28, in <module>
    spitfire.runtime.template.enable_psyco(ambiguous_in)
  File "/Users/nicksay/dev/spitfire/spitfire/runtime/template.py", line 83, in enable_psyco
    import psyco
ImportError: No module named psyco

Fix this.