Added in --expose-gc flag

Also added in --build-only stub, will fill in later
Crank up the timeout
Modified revwalk test to only call done once
stop thrashing cpu with GCs
Adds in a BUILD_ONLY environment variable

By default the module will attempt to install from a prebuilt binary
from S3.  If this fails it will attempt a native compile.

For our continuous integration and local development we'll always run
with `BUILD_ONLY=true`.
More advanced test script

Can determine environment and which flags should be set.  Will fix
expose-gc bug in Windows.

Test coverage reports have been moved to the test folder and the
gitignore is updated accordingly.
Fixed tests running on windows
Merge pull request #287 from nodegit/better-install-process

[WIP] Better installation flow
add history.md and update readme