[Fix #330, #322] scatter: single scatter non default size

Similar to my previous pull request, just re-did the pull properly (?).

Note, the issues arised in the previous code if there was a single point. In that case it was never re-scaled even if the user set a specific size. Here, I just resize every time that one size is specifie, even if default (to be sure, also cleaner code).

The fix produces:

![fix330](https://cloud.githubusercontent.com/assets/3731173/4774740/3cfb1c0a-5bb0-11e4-83b0-32dd6a3a3234.PNG)

compare with
![unfixed](https://cloud.githubusercontent.com/assets/7280989/2689758/fa4b5104-c31b-11e3-8727-61dad81305fe.png)


Aslo, I re-introduced partly the changes by @nidrosianDeath. Somehow his work got lost, but even I cannot reproduce teh result in his [comment](ttps://github.com/nschloe/matlab2tikz/pull/316#issuecomment-35383893), using 

    sData = translateMarkerSize(m2t, matlabMarker, sqrt(sData)/2);

already improves the situation.

Follow a bunch of tests that show how it improves *line marker size unaffected):

![linemarkersize](https://cloud.githubusercontent.com/assets/3731173/4774759/70e5b67e-5bb0-11e4-8af5-491e85d5a48f.PNG)
![test62-scatter](https://cloud.githubusercontent.com/assets/3731173/4774763/79299288-5bb0-11e4-8ea0-545c7349d6b9.PNG)
![scattermarkersize](https://cloud.githubusercontent.com/assets/3731173/4774764/7b9910ac-5bb0-11e4-8cc6-7a6e963b2601.PNG)

scatter Marker Sizes improved as per the discussion in #316