The code in this PR finds the necessary mirgations, downloads the latest version of them from the distribution site, unpacks the executables, and runs the migrations in order.

This code is also used to build the ipfs-update tool and the fs-repo-migrations tool.  Note: the fs-repo-migrations tool is only used to run stand-alone migrations now and is not used by either go-ipfs or ipfs-update.

Additional utility is provided by this PR, that is not specific to migrations:
- Find local ipfs directory
- Get current repo version
- Check for ipfs daemon availability
- Get version information about any distribution on distribution site
- Fetch and unpack any binary executable over ipfs or http

Since the migrations are not displayed on the dirtributions, there is not need to organize them under their own root to reduce visual clutter.  Having each migration follow the same path as all other distributions makes each easier to find in the absence of a link displayed on the distributions web page.  It also avoids complicating the distribution deployment scripts and allows each migration distribution to be treated the same as any other distribution.

- IpfsDir gets the location of the ipfs directory, whether it exists or not
- CheckIpfsDir get the location and checks whether it exists.

…since that breaks tests that get the version to see if ipfs is initialized

- Udate path to IPFS dist
- Improve test coverage

* rename migrations from ipfs-x-to-y to fs-repo-x-to-y
* update current ipfs dist to one with migrations v1.0.0

… in .exe

… does not support read-only directories