Low memory

[olgabot]

In running outrigger psi or other command line programs, I get these errors:

/home/obotvinnik/workspace-git/outrigger/outrigger/commandline.py:937: DtypeWarning: Columns (18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,42,43,44,45,46,47,48,49,50,51) have mixed types. Specify dtype option on import or set low_memory=False.
  cl = CommandLine(sys.argv[1:])

Since I expect most users of outrigger to use it on a large dataset, on a supercomputer, this PR sets low_memory=False by default, but can be turned off with the command line flag --low-memory

• [x] The pull request should include tests.
• [x] If the pull request adds functionality, the docs should be updated. Put your new functionality into a function with a docstring, and add the feature to the list in README.md and README.rst.
• [x] The pull request should work for Python 2.7, 3.4, and 3.5.

[willingc]

@olgabot I did a quick code review on this. It looks good to me.

[olgabot]

Yep, just gotta add some release notes and maybe a test.