ENH: better control of mirroring in Text components; + changelogs

- can now do dynamically using $var_name, e.g., from a conditions file
- simpler dialog box (enter 'horiz', 'vert', or '$var_name')

(I'll use this feature soon for a mental rotation task.)
BF: refactor flipVert flipHoriz for TextStim and BufferImageStim

- aims for usage consistent with SimpleImageStim
RF: refactor Text component for flipHoriz/Vert

known limitations:
- TextStim.setFlip() looks redundant but simplifies the Builder dialog
  (no need for separate dialog entries for flipHoriz, flipVert)
- I could not get 'set every repeat' to work ('set every frame' seems ok)
Merge branch 'master' of git://github.com/psychopy/psychopy

* 'master' of git://github.com/psychopy/psychopy:
  DOCS: minor tweaks/typos
  BF: SoundPyo.sampleRate should come from server not from SoundPyo.__init__()
DOC: coder demo style-guide (esp for OpenHatch event)

anything to add to this? tweak?
how about explicitly saying in each code demo that its public domain?
Merge pull request #253 from jeremygray/master

allow a variable to control mirroring for text components