Better meta for 'text' keys:

- Annotation 'text' includes a description of HTML tags and a few examples
- 'text' in now included in Histogram
- better description for 'text' (Scatter/Bar/Histogram), 'textpostion' and 'textfont'
- example for 'scl' is now shorter, to fit on webpage.
- available font family match revised list (refer to  GUI)
Merge pull request #12 from plotly/newline-discussion

Better meta for 'text' keys