Revamp setup

Make setup.py simpler and more robust by:

* using versioneer
* removing the cranky ez_setup.py
* specifying all dependencies as `install_requires`
* specifying minimum versions of all dependencies
* using tifffile from PyPI, and removing it from `extern/` (grep confirms that all references to `extern` have been removed)

The bad outweighs the good with ez_setup.py, especially now that we will encourage users to use conda builds:

    conda install -c soft-matter pims

Also:

* Credit "PIMS contributors" (it was just my name, from early days)
* Stealing a trick from trackpy, use the full README.md as the `long_description`.