s/WITH_PNACL/WITH_NATIVE_CLIENT/ in a few places

This prevented specifying GPU targets with PNaCl targets
Merge pull request #553 from halide/WITH_NATIVE_CLIENT

s/WITH_PNACL/WITH_NATIVE_CLIENT/ in a few places
EXPORTed operator<<(ostream, Stmt)
Added EXPORT to another defininition of operator<<(ostream, Stmt)
Added get_target_triple to CodeGen_PNaCl.
PNACL -> PNaCl
Move NamesInterface from Internal to Halide namespace
Merge branch 'master' of https://github.com/halide/Halide
Add Generator::get_filter_parameters()
Merge branch 'master' of https://github.com/halide/Halide
Change int -> size_t in GeneratorBase::get_filter_parameters()
Update test_all_targets.sh to use libpng1.6.14
Parameter::get_scalar() should be const
Ensure that there is a DataLayout pass on the LLVM pass list for
optimization in LLVM 3.6. This prevents the crash we've been seeing in
optimize_module and mirrors the behavior of LLVM's opt tool.
Ensure that modules always have a data layout.
Parameter::get_min/max_value should be const
Fix issue #561 (bogus "Invalid Param name" error)
Revert "Fix issue #561 (bogus "Invalid Param name" error)"

This reverts commit e3dbd7367c40d72e036dd3b9709d897ed61a9606.
Take 2: Fix issue #561 (bogus "Invalid Param name" error)

First attempt was both broken and ill-conceived.
Add Parameter::operator=