Dynamic path for composer task

[ghost]

It would be a nice feature to have the phing composer task check if a composer installation is available to execute the command with if no composer executable path is set.

[mrook]

Couple of comments:

• this introduces whitespace changes (we indent using 4 spaces)
• which is not available on Windows

[mrook]

Related to #558.

[ghost]

Would it be a good idea to implement a try catch on the command with the provided composer path and then try finding a path if that fails? That would cover the user entering a wrong path. But then the user would not be aware of that mistake.

[ghost]

I moved the path check to the getComposer function and cleaned up the class a bit.

[ghost]

Mind giving me a new review @mrook ?

[mrook]

Can you squash the commits? The PR is a little noisy this way :)

Anyway:

• I see a lot of unrelated changes (docblock modifications etc.)
• Instead of checking against php_uname you can perhaps reuse the host.fstype property