prevent NAT check if given nil multiaddr

Install sharness

It is cleaner if sharness is installed in its own
directory.

Install sharness + add to tests to coverage

Addresses comments in: https://github.com/jbenet/go-ipfs/commit/7e14f1737d146cb45f3d788f229dae9fd964e524

The tests were using the globally installed ipfs.
This commit changes it to build + use its own ipfs binary.

When there is no fuse we should skip all mount
tests and using a FUSE prerequisite is a good
way to do that.

Fix buffer size bug in fuse mounts

sha1sum is not in osx by default.
shasum seems to be everywhere.

Miscellaneous fixes

Add test for ipfs mount

License: MIT
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>

should be able to look at a function in isolation and prove it won't
panic. if that's not possible, should cast safely.

use case:

Just configured and installed a node.

benefits:

1) reduces friction when setting up a new node

2) reveals useful details about the workings of the system. It's the
user's first encounter with her node's identity. The tour can build on
the user's knowledge.

```
ipfs (maybebtc-november) λ. ipfs init -f
initializing ipfs node at /Users/btc/.go-ipfs
generating key pair
peer identity: QmcRbn41Vc2CvbpLYfN36mAWusErKWvAAHbq92LPra2gFT
```

Fixes #258

https://github.com/jbenet/go-ipfs/issues/258

add context to DialPeer interface

@jbenet