handle empty string as none

[ericman93]

When VCF info field is empty string, add None to info dict

[martijnvermaat]

Cool, thanks!

Could you make the same change in cparse.pyx?

I see a few possible improvements for the test:

1. Add the test case to the suite at the bottom of test_vcf.py (I'm sorry, this is not an optimal setup, but otherwise your test won't actually be executed by a standard python setup.py test).
2. In the test VCF file, define the INFO fields in the header.
3. Also use a flag INFO field (FLAG;EMPTY=;DOT=.;NOTEMPTY=6)
4. Include a unary and n-ary variant of each field (EMPTY_1, EMPTY_N, DOT_1, DOT_N, etc), just like the currently unused fields in the header.

[martijnvermaat]

I made the tests slightly more comprehensive and merged this.

Thanks!