Caching for type information

Hi Luke,

Can you consider pulling from my fork? I added a feature to store GHCi generated type information in a cached 
file. This speeds up the type information and omni-completion commands and makes them much more usable 
when used with big modules such as OpenGL. Now they don't have to consult GHCi for type information every 
time a file is reopened. It is left up to the user to reload the type information from GHCi using GHCReload. 
Otherwise the cached information is used whenever available.

I also updated the documentation and fixed a minor issue in the initialization of the b:my_changedtick
variable.

Best Regards,
Tommi Johtela