actually check status values from all HDF5 calls
check if window file is empty in WindowDataLayer

Also note that window files containing windows with different numbers of
channels may not work correctly.
initialize in declared order
initialize in declared order in tests
remove unused variables
remove unused variables from tests
add missing const qualifiers to MemoryDataLayer ExactNum* functions
move CUDA 6.0 check into switch statement itself

This allows -Wswitch to be turned on so that the compiler can check
exhaustiveness.
initialize and comment variables that the compiler finds suspicious
don't end comments with \, so that -Wcomment can be used
upgrade warnings to -Wall -Werror -Wno-sign-compare
add WARNINGS to CXXFLAGS
turn off some warnings for older compilers
fix test data layer post-lmdb
make clang++ happy on OSX by not linking with pthread
content ourselves to -Wall without -Werror for now
Merge pull request #488 from longjon/wall-werror

Build with -Wall -Werror