Merge branch 'release/0.6.3' into develop
Next version is 0.7.0
Added a pip requirements file (primarily for readthedocs.org)
examples/demo_struct_reduce: added a docstring
examples: added a simple transformation demo
doc/history: mentioned the added example
fft: added a type check in the constructor

Better to get a clear error right away, than a giant stack trace during compilation.
doc/history: updated for the previous commit
Make Program a new-style class so that repr works correctly
Merge pull request #18 from ringw/print-program-fix

Fixed exception caused by printing CUDA program object
doc/history: documented a recent fix
doc/history: fixed dates
test/helpers: using numpy.allclose() to check the equality of arrays in tests
test/helpers: improved diff_is_negligible() output
test: relaxed tolerances in some tests due to the change to per-element comparison with the reference
cluda/functions: added output_dtype parameter for pow() and renamed power_dtype to exponent_dtype
cluda/functions: fixed a corner case in pow()
cluda: Array objects now have `thread` attribute
doc/history: fixed syntax
v0.6.4 released