Made CategoryEncoder.builder() concrete, as in other encoders.
Fix for #86. Added MultiEncoder>
Added PassThroughEncoder.Builder.
Added SparsePassThroughEncoder.Builder.
Merge branch 'add_multiencoder' of https://github.com/wlmiller/htm.java into wlmiller-add_multiencoder

Conflicts:
	src/test/java/org/numenta/nupic/encoders/PassThroughEncoderTest.java
	src/test/java/org/numenta/nupic/encoders/SparsePassThroughEncoderTest.java
Merge branch 'wlmiller-add_multiencoder'
Merge pull request #128 from cogmission/master

Merging work and resolving conflicts for @wlmiller