Fix cheetah_cheats mode to resolve local placeholders
Fix cheetah_cheats mode to resolve local placeholders

When compiled with `-Xcheetah-cheats` (or when `options.cheetah_cheats`
is `True`) and no optimizations are applied, the template source
```
  #set $a = 'foo'
  #set $b = $a
```
produces the template code
```
  _self_search_list = self.search_list + [_globals]
  a = u'foo'
  b = resolve_placeholder(_self_search_list, 'a')
```
which causes a runtime error.

Fix this by adding `locals()` to the search list during
placeholder resolution.  Also fix a bug where a runtime error
occurs if `self.search_list` is `None` by conditionally adding
it to the search list.  After, the template code is
```
  if self.search_list:
    _self_search_list = self.search_list + [_globals]
  else:
    _self_search_list = [_globals]
  a = u'foo'
  b = resolve_placeholder([locals()] + _self_search_list, 'a')
```

Closes #6

spt/spitfire,nicksay/spitfire,spt/spitfire,spt/spitfire,youtube/spitfire,youtube/spitfire,youtube/spitfire,youtube/spitfire,nicksay/spitfire,nicksay/spitfire,spt/spitfire,nicksay/spitfire