IndividualField: add operator== and operator!=

Add implementation of the two operators for both IndividualField and
IndividualFieldValue so we can write tests more easily and compare
objects. Comparison is based on value equality. The Genotype class
already had the operators implemented (thanks @kshakir)

fixes #187
Merge pull request #198 from broadinstitute/mc_individual_operator_equals_187

IndividualField: add operator== and operator!=