Added possibility to cache GHC type information.

Added a new experimental feature that allows storing the GHC symbol
data to a background file. This data is used in the _t and balloon
commands to show the type information for a symbol.

A hidden file with a same name as the original haskell file but
with the ".sym" extension is used to store the type dictionary. When
this file is available it is used to populate the ghc_types
dictionary. It is still possible to reload the symbol information
from GHCi using the :GHCReload command.

The feature is enabled by setting the g:ghc_symbolcache variable.
If this variable is not defined, the symbol information management
should work as before.
Fixed the overflown line in the help file.
Fixed the my_changedtick initialization problem

The my_changedtick should be initialized to -1 in order to
load the symbol table on the first lookup. Otherwise type
information command (_t) does not find the symbol before either
balloon or GHCReload function is called.
Merge pull request #11 from johtela/master

Caching for type information