Fix project persistency implementation

[suoto]

Using Pickle to dump/load project objects to a file causes issues when the object is dumped in a Python shell and loaded from another shell that doesn't have access to the objects the original shell had.

For example, building a project within Vim then trying to build it via command line gives the following exception

Traceback (most recent call last):
  File "/home/asouto/bin/runner.py", line 213, in <module>
    main(runner_args)
  File "/home/asouto/bin/runner.py", line 161, in main
    project.readConfigFile()
  File "/home/asouto/utils/vim/vim/bundle/vim-hdl/python/vimhdl/project_builder.py", line 79, in readConfigFile
    obj = pickle.load(open(cache_fname, 'r'))
  File "/home/asouto/utils/vim/vim/bundle/vim-hdl/python/vimhdl/vim_client.py", line 25, in <module>
    import vim
ImportError: No module named vim

[suoto]

Pickle-like serializers dump whole environment definition and expect the environment to remain somewhat the same and this can't be guaranteed.

This approach will be replaced by using JSON to cache only the required attributes. A good side effect is that since class definitions won't be cached, reusing the same cache with different hdlcc versions should be easier.

[suoto]

Results using JSON seem OK, need more throughout testing.

[suoto]

Fixing this issue should fix issue #2 as well.