Preliminary API changes towards reducing special casing of subject/label

Remove subject/label methods from the API, replace some methods with general
methods taking a property name as an argument.

This breaks quite a few things
Further changes to label/subject

Remove global label/subject lookup table
Consistently use "Subject" rather than "Label", subject is assumed to be
convertable to QString. When desirable, map discrete subject values to ints.

For classifiers such as svm that require numeric labels, generate a string->int
mapping for the training data, and store it (local to the transform).

Utility functions for collecting all values of a given property (on a template
list), and mapping discrete property values to 0-based integers

Some outstanding issues include use of label/subject in matrix output
Merge branch 'master' of https://github.com/biometrics/openbr into local_remap
Fix evalClassification output
Rename collectValues to get, add a values method

These names mirroring File functionality.
Fixed SVMDistance
dropped unnecessary second relabel in LDATransform::train
Remove special casing of "Subject" in MatrixOutput::toString
Remove some special casing of label and some dated comments

Remove special casing of label in cache, and getCSVElement
Drop pixel.cpp, it is currently unmaintained (and broken by recent changes)
Merge branch 'master' of https://github.com/biometrics/openbr into local_remap
Update models ref
Merge pull request #53 from biometrics/local_remap

Changes to subject/label