Merge pull request #92 from sunfishcode/master

Int16x8 and Int8x16 support
Remove unnecessary 'new' keywords
Use strict equality checks for testing for undefined
Indentation fixes
Fix copy+pastos in comments
Rename _PRIVATE to _SIMD_PRIVATE
Avoid automatic semicolon insertion
Refactor helper functions

Eliminate duplicate copies of helper function code, and place helper
function code in the _SIMD_PRIVATE namespace.
Use coercion constructors instead of check functions

Coercion constructors are expected to be the recommended way for most
code to ensure that variables are of the expected SIMD types, so use
that style in the polyfill when it makes sense too.
Use '.x' rather than 'x_' when appropriate

Library user code will use the accessors like '.x' rather than accessing
the members like '.x_' directly, so make more code do that. This also
makes the code easier to drop into a polyfill for a partial native
implementation. It also makes it easier to experiment with alternate
value representations in the polyfill.
Integrate with partial native implementations

Wrap each declaration in a test for an existing declaration, so that the
polyfill can be transparently used in conjunction with a native
implementation.