Add touch() fn to File primitive

[itpetey]

Currently the only way to create a new file is to upload one from the local disk. In some cases it would be useful to be able to create a blank file directly on the remote machine.

The function signature will be:

impl File {
//...
    /// Create a blank file on the managed host.
    fn create(&self, host: &mut Host) -> Result<()>;
//...
}

There are several places that this function will need to be added to:

1. file/mod.rs -> impl File
2. file/mod.rs -> trait FileTarget
3. file/ffi.rs -> pub extern "C" fn file_create(...)
4. All implementation of FileTarget in target/...
5. Agent repo -> api/file.rs
6. Examples repo -> Projects for Rust, C and PHP
7. Tests repo -> Test suites for Rust, C and PHP