Added approximation of text alignment. Better then nothing...

This should be a first approach to issue #17.

I used the SVG standard sections on [text-anchors][1] and [baseline alignment][2] to find what should be closest. I could not find properties to modify the alignments in a free manner as given by `BoxAlignedText`.

While I looked at the [user manual][3] I noticed that according to [documentation][4] shouldn't the `(0.7,0.5)` text displayed in the manual be right aligned instead of left aligned since 1 means right and 0 means left? Or am I misunderstanding the semantics here?

[1]: http://www.w3.org/TR/SVG/text.html#TextAnchorProperty
[2]: http://www.w3.org/TR/SVG/text.html#BaselineAlignmentProperties
[3]: http://projects.haskell.org/diagrams/doc/manual.html#text
[4]: http://hackage.haskell.org/packages/archive/diagrams-lib/0.7/doc/html/Diagrams-TwoD-Text.html#v:alignedText