Asyncprogress

Rebased and fixed. Replaces #175 and #181. Closes #169.
One thing stands out: Dicked around with string converters ed44ec61a8473d56cd225cf3c6d086d1457d959c deprecated the undocumented Size() method of the string converters in favor of length(), to match v8's converters. Both Size() and length() now return the same value, which is 1 less than what Size() previously returned (this could be rectified with one more patch if necessary to keep the old behavior...)

feature: added NAN_GC_CALLBACK 6a5c24505f0cad97a8a958039cd64406e8c6c27f
performance: Removed unnecessary local handle creation 18a7243863dcb393fcb6de9af9e2f33e505a8a9c, 41fe2f8e8df746e2e3d44611954e6c72457a2659
correctness: Added const-ness to references in NanHasInstance 02c61cdb0ced169e34de8b665aa853301c0fdf24
warnings: Fixed spurious warnings from -Wundef and -Wshadow, 541b122b9a30eb32ef2745142436e3dac0dcf94e, 99d8cb6a4ab27fdc7eeaea87f22974070f7a633c
windoze: Shut Visual Studio up 8d558c1631047d5e55b590e6abd58cbe78a4e372
license: switch to plain MIT 11de98307741bffcc4c0da5204ded25e217e94ef
build: Added test target to Makefile e232e46da6a40aec8e974a61f739971c40f7e1c7
whatever: Dicked around with string converters ed44ec61a8473d56cd225cf3c6d086d1457d959c
performance: Removed superfluous scope in NanAsyncWorker f4b78218b3d83616378fb6bcc9f0f6c349683dcc
sugar: Added NanReturnThis() and NanReturnHolder() shorthands 237a5ff6a4bfd03aa2dd1f7f894c0e0791f12c8d, d6972084d3e927d1ddb134b37d9b2540f122342f
awesomeness: Added suitable overload of NanNew for v8::Integer::NewFromUnsigned b27b4500b4ad747a78183a1af468a07901981785