Solar radius, SoHO and distances - Not to merge!!! yet.

This is a couple of changes I'm proposing after the [discussion we had today on the mailing list](https://groups.google.com/d/msg/sunpy-dev/W6nETQRsC7w/hdRHkmxxYD4J) and this pull request is mainly for discussion on how we handle this issue.

I've started to wonder whether we need the SOHO keyword (or something of the kind) or not, so when we compare images from different spacecraft it does the conversion of arcsecs properly.  Because 1 arcsec from Earth is not the same that 1 arcsec from SoHO.
![rsun](https://f.cloud.github.com/assets/963242/392375/4e6f248e-a77c-11e2-9919-85fe8e66468f.png)

dsun (a parameter in map) should take care of this, so in principle the SOHO keyword would not be needed. But dx and dy should change to a singular point of view when comparing different maps. 

The changes I've made are mainly on the angular_size function which gives just half-sun and it could be misleading as for me the angular_size of the sun is 30 arcminutes, this is, the whole Sun.  This change of name could break few things...

Also, I've used constant.radius, that it has a reference to a paper, instead of a value that we don't know whether comes from: 959.63 which it's used in [sswidl](http://hesperia.gsfc.nasa.gov/ssw/gen/idl/solar/get_sun.pro) and it's mentioned [in a page I've found](http://astro.sci.muni.cz/pub/hollan/a_papers/working/sun_rad.htm).

Now, the big debate! do we do our way or we follow SSW?