Fixed clang support

Clang has very bad support for local classes, so just use normal classes
instead. Now gcc and clang are both fully supported.
Minor compatibility fixes
Added task_handle conversion to and from void pointer.

This allows task_handle objects to be passed through interfaces
which don't preserve types.
Added build scripts for clang.
Moved documentation to the wiki. Replaced README with a short example.
Added support for MinGW. Requires GCC 4.8+ with posix threads.
Updated README
Fix typo
Replace VLA with a vector
Workarounds for Clang (Apple XCode version 3.1)
Fix compile error
Force symbol visibility to hidden unless explicity exported.
Added build scripts to compile with clang and libc++.
Add __has_feature check for thread_local keyword support.
Updated README
Fixed example in README

Lambdas with multiple statements must have a return type specified.
Refactored task classes to remove fragile enable_if logic.
Fixed check for thread_local: GCC doesn't support __has_feature.
Refactored helper functions for handling void tasks
Fixed execution functions to avoid redundant code generation.