Moved some files/tests to respective subpackages

This is only the file moves without changes; this commit will not
compile.
Moved test yaml files and walker core tests

Realized that if helpers is going to depend on walker, then the walker
tests can't be in the walker package because they would depend on
helpers (circular). Moving the core walker tests back in to test/.
Package name fixes after move to subpackages
Fixed export symbols etc. for subpackage move
Added test/doc.go, partly to make travis ci happy

Travis requires `go get -t -v ./...` to work, which it doesn't for
packages that only have test files.
Fixed script/coverage.sh to handle new subpackages
Merge pull request #54 from iParadigms/subpackage-moves

Subpackage moves