Linear Regression: Fix to multiple item drop crash

Dropping multiple variables at the same time into the 'Blocks' field
would crash JASP.
ANOVA: Fix to remove "Is Nuisance" label
Adjustments to the Terms object

Changed insertion behaviour, and added an ::asString() method which can
be useful when debugging
Linear Regression: Fix to problem dragging #46

Fixes #46
Linear Regression: Adjustment to available list

The available variable list now greys out Nominal Text variables
Merge pull request #56 from mmarsman1/development

fix #49, fix #54
Updates to OS X build notes

Included the necessity of installing XCode
Fixed a bug with na-handling