add text/html writemime for MethodList and Method (fix #4952)

The main point of this, besides a bit of pretty-printing, is to automatically link to github for Julia and Pkg source code (#4952).

Sample output:
![methodshow](https://f.cloud.github.com/assets/2913679/1649547/1f29890a-5a0d-11e3-9a8c-873db2c879a1.png)