Add assertions for files and directories

[ddidier]

A very useful project, thank you.
Some new assertions to consider:

• assertFileExists
• assertFileDoesNotExist
• assertFileContains
• assertFileDoesNotContain
• assertDirectoryExists
• assertDirectoryDoesNotExist

[ddidier]

See pull request #115

[cbandy]

test supports many expressions. Why single these few out?

The recently added failFound and failNotFound might be what you're looking for:

[ -f want ] || failNotFound want
[ ! -f dont ] || failFound dont

grep -q needle /haystack || failNotFound ...

[ddidier]

I didn't see this pattern, but I believe the ones I added are more readable (and I use them often).