Make __init__.py use a relative import

[bpteague]

Instead of making cytoflow depend on fcsparser, I would like to include fcsparser as a submodule. This way, if I have fixes that I want to roll out with a cytoflow release, I don't have to wait on you to roll a new release of fcsparser. (Making new releases is harder than it should be.)

This would be a lot easier on my end if fcsparser.__init__ used a relative import instead of an absolute import. This way, the directory containing api.py doesn't have to be in sys.path. It should not effect folks who use fcsparser directly.

[eyurtsev]

Thanks! I think it's generally advisable to use relative imports in python packages that are to be distributed